[ELF][PPC64] Enable R_PPC64_REL14 trunks

[MaskRay]

Summary

The thunk implementation is available but an assertion disallows it.
Linux kernel has such a use case: in arch/powerpc/kernel/exceptions-64s.S:handle_page_fault,
beq+ ret_from_except_lite may get out of range.

Link: https://github.com/ClangBuiltLinux/linux/issues/951

[MaskRay]

Rebase after D76907

lld/ELF/Thunks.cpp

assert((rel.type == R_PPC_LOCAL24PC || rel.type == R_PPC_REL24 ||
           rel.type == R_PPC_PLTREL24) &&
          "unexpected relocation type for thunk");
   if (s.isInPlt())
     return make<PPC32PltCallStub>(isec, rel, s);
   return make<PPC32LongThunk>(s, rel.addend);
 }
 
 static Thunk *addThunkPPC64(RelType type, Symbol &s, int64_t a) {
-  assert(type == R_PPC64_REL24 && "unexpected relocation type for thunk");
+  assert((type == R_PPC64_REL14 || type == R_PPC64_REL24) &&
+         "unexpected relocation type for thunk");
   if (s.isInPlt())
     return make<PPC64PltCallStub>(s);
 
   if (config->picThunk)
     return make<PPC64PILongBranchThunk>(s, a);
 
   return make<PPC64PDLongBranchThunk>(s, a);
 }

lld/test/ELF/ppc64-long-branch-rel14.s

+# REQUIRES: ppc
+# RUN: echo 'SECTIONS { \
+# RUN:       .text_low 0x2000: { *(.text_low) } \
+# RUN:       .text_high 0xa000 : { *(.text_high) } \
+# RUN:       }' > %t.lds
+
+# RUN: llvm-mc -filetype=obj -triple=powerpc64 %s -o %t.o
+# RUN: ld.lld -T %t.lds %t.o -o %t
+# RUN: llvm-readelf -r %t | FileCheck --check-prefix=SEC %s
+# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
+
+# RUN: llvm-mc -filetype=obj -triple=powerpc64le %s -o %t.o
+# RUN: ld.lld -pie -T %t.lds %t.o -o %t
+# RUN: llvm-readelf -r %t | FileCheck --check-prefix=SEC %s
+# RUN: llvm-objdump -d --no-show-raw-insn %t | FileCheck %s
+
+# SEC: There are no relocations in this file.
+
+# CHECK-LABEL: <_start>:
+# CHECK-NEXT:  2000: bt 2, .+20
+# CHECK-NEXT:        bt+ 2, .+16
+# CHECK-NEXT:        bf 2, .+32764
+# CHECK-NEXT:        bt 2, .+24
+# CHECK-NEXT:        blr
+# CHECK-EMPTY:
+# CHECK-NEXT: <__long_branch_high>:
+# CHECK-NEXT:  2014: addis 12, 2, 0
+# CHECK-NEXT:        ld 12, {{.*}}(12)
+# CHECK-NEXT:        mtctr 12
+# CHECK-NEXT:        bctr
+# CHECK-EMPTY:
+
+# CHECK-NEXT: <__long_branch_>:
+# CHECK-NEXT:  2024: addis 12, 2, 0
+# CHECK-NEXT:        ld 12, {{.*}}(12)
+# CHECK-NEXT:        mtctr 12
+# CHECK-NEXT:        bctr
+
+.section .text_low, "ax", @progbits
+.globl _start
+_start:
+beq high           # Need a thunk
+beq+ high          # Need a thunk
+bne high
+beq .text_high+16  # Need a thunk
+blr
+
+# CHECK-LABEL: <.text_high>:
+# CHECK-NEXT:      a000: nop
+# CHECK-EMPTY:
+# CHECK-LABEL: <high>:
+# CHECK-NEXT:      a004: bf 0, .+32772
+# CHECK-NEXT:            bt 1, .+32768
+
+.section .text_high, "ax", @progbits
+nop
+.globl high
+high:
+bge .text_low+8
+bgt .text_low+8