[AMDGPU] Add RELATIVE64 relocation record.

[t-tye]

Summary

Add base relative relocation record that can be used for the following case (OpenCL example):

static __global int Var = 0; 
__global int* Ptr[] = {&Var};
...

In this case Var is a non premptable symbol and so its address can be used as the value of Ptr, with a base relative relocation that will add the delta between the ELF address and the actual load address. Such relocations do not require a symbol.

[kzhuravl]

LGTM.

docs/AMDGPUUsage.rst

**GOT**
   Represents the address of the global offset table.
 
 **P**
   Represents the place (section offset for ``et_rel`` or address for ``et_dyn``)
   of the storage unit being relocated (computed using ``r_offset``).
 
 **S**
   Represents the value of the symbol whose index resides in the relocation
-  entry.
+  entry. Relocations not using this must specify a symbol index of ``STN_UNDEF``.
+
+**B**
+  Represents the base address of a loaded executable or shared object which is
+  the difference between the ELF address and the actual load address. Relocations
+  using this are only valid in executable or shared objects.
 
 The following relocation types are supported:
 
   .. table:: AMDGPU ELF Relocation Records
      :name: amdgpu-elf-relocation-records-table
 
      ==========================  =====  ==========  ==============================
      Relocation Type             Value  Field       Calculation
      ==========================  =====  ==========  ==============================
      ``R_AMDGPU_NONE``           0      *none*      *none*
      ``R_AMDGPU_ABS32_LO``       1      ``word32``  (S + A) & 0xFFFFFFFF
      ``R_AMDGPU_ABS32_HI``       2      ``word32``  (S + A) >> 32
      ``R_AMDGPU_ABS64``          3      ``word64``  S + A
      ``R_AMDGPU_REL32``          4      ``word32``  S + A - P
      ``R_AMDGPU_REL64``          5      ``word64``  S + A - P
      ``R_AMDGPU_ABS32``          6      ``word32``  S + A
      ``R_AMDGPU_GOTPCREL``       7      ``word32``  G + GOT + A - P
      ``R_AMDGPU_GOTPCREL32_LO``  8      ``word32``  (G + GOT + A - P) & 0xFFFFFFFF
      ``R_AMDGPU_GOTPCREL32_HI``  9      ``word32``  (G + GOT + A - P) >> 32
      ``R_AMDGPU_REL32_LO``       10     ``word32``  (S + A - P) & 0xFFFFFFFF
      ``R_AMDGPU_REL32_HI``       11     ``word32``  (S + A - P) >> 32
+     *reserved*                  12
+     ``R_AMDGPU_RELATIVE64``     13     ``word64``  B + A
      ==========================  =====  ==========  ==============================
 
 .. _amdgpu-dwarf:
 
 DWARF
 -----
 
 Standard DWARF [DWARF]_ Version 2 sections can be generated. These contain

include/llvm/BinaryFormat/ELFRelocs/AMDGPU.def

 #ifndef ELF_RELOC
 #error "ELF_RELOC must be defined"
 #endif
 
 ELF_RELOC(R_AMDGPU_NONE,           0)
 ELF_RELOC(R_AMDGPU_ABS32_LO,       1)
 ELF_RELOC(R_AMDGPU_ABS32_HI,       2)
 ELF_RELOC(R_AMDGPU_ABS64,          3)
 ELF_RELOC(R_AMDGPU_REL32,          4)
 ELF_RELOC(R_AMDGPU_REL64,          5)
 ELF_RELOC(R_AMDGPU_ABS32,          6)
 ELF_RELOC(R_AMDGPU_GOTPCREL,       7)
 ELF_RELOC(R_AMDGPU_GOTPCREL32_LO,  8)
 ELF_RELOC(R_AMDGPU_GOTPCREL32_HI,  9)
 ELF_RELOC(R_AMDGPU_REL32_LO,      10)
 ELF_RELOC(R_AMDGPU_REL32_HI,      11)
+ELF_RELOC(R_AMDGPU_RELATIVE64,    13)

test/Object/AMDGPU/elf64-relocs.yaml

 # CHECK:     0x8  R_AMDGPU_REL32          - 0x0
 # CHECK:     0x10 R_AMDGPU_REL64          - 0x0
 # CHECK:     0x12 R_AMDGPU_ABS32          - 0x0
 # CHECK:     0x14 R_AMDGPU_GOTPCREL       - 0x0
 # CHECK:     0x16 R_AMDGPU_GOTPCREL32_LO  - 0x0
 # CHECK:     0x18 R_AMDGPU_GOTPCREL32_HI  - 0x0
 # CHECK:     0x20 R_AMDGPU_REL32_LO       - 0x0
 # CHECK:     0x22 R_AMDGPU_REL32_HI       - 0x0
+# CHECK:     0x24 R_AMDGPU_RELATIVE64     - 0x0
 # CHECK:   }
 # CHECK: ]
 
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
Relocations:
         Symbol:          s9
         Type:            R_AMDGPU_GOTPCREL32_HI
       - Offset:          0x20
         Symbol:          s10
         Type:            R_AMDGPU_REL32_LO
       - Offset:          0x22
         Symbol:          s11
         Type:            R_AMDGPU_REL32_HI
+      - Offset:          0x24
+        Symbol:          s12
+        Type:            R_AMDGPU_RELATIVE64
 
 Symbols:
   Local:
     - Name:            .text
       Type:            STT_SECTION
       Section:         .text
 
   Global:
     - Name:            main
       Type:            STT_FUNC
       Section:         .text
       Size:            0x08