[MSP430] Use R_MSP430_16_BYTE type for FK_Data_2 fixup

[krisb]

Summary

Linker fails to link example like this (simplified case from newlib
sources):

$ cat test.c

extern const char _ctype_b[];
struct _t { char *ptr; };
struct _t T = { ((char *) _ctype_b + 3) };

$ cat ctype.c

char _ctype_b[4] = { 0, 0, 0, 0 };

LD: test.o:(.data+0x0): warning: internal error: unsupported relocation error

We also follow gnu toolchain here, where 2-byte relocation mapped to
R_MSP430_16_BYTE, instead of R_MSP430_16.

llvm/trunk/lib/Target/MSP430/MCTargetDesc/MSP430ELFObjectWriter.cpp

public:
   ~MSP430ELFObjectWriter() override {}
 
 protected:
   unsigned getRelocType(MCContext &Ctx, const MCValue &Target,
                         const MCFixup &Fixup, bool IsPCRel) const override {
     // Translate fixup kind to ELF relocation type.
     switch ((unsigned)Fixup.getKind()) {
     case FK_Data_1:                   return ELF::R_MSP430_8;
-    case FK_Data_2:                   return ELF::R_MSP430_16;
+    case FK_Data_2:                   return ELF::R_MSP430_16_BYTE;
     case FK_Data_4:                   return ELF::R_MSP430_32;
     case MSP430::fixup_32:            return ELF::R_MSP430_32;
     case MSP430::fixup_10_pcrel:      return ELF::R_MSP430_10_PCREL;
     case MSP430::fixup_16:            return ELF::R_MSP430_16;
     case MSP430::fixup_16_pcrel:      return ELF::R_MSP430_16_PCREL;
     case MSP430::fixup_16_byte:       return ELF::R_MSP430_16_BYTE;
     case MSP430::fixup_16_pcrel_byte: return ELF::R_MSP430_16_PCREL_BYTE;
     case MSP430::fixup_2x_pcrel:      return ELF::R_MSP430_2X_PCREL;

llvm/trunk/test/MC/MSP430/reloc.s

; CHECK:                        ;   fixup A - offset: 2, value: disp+2, kind: fixup_16
          mov    disp, disp+2
 ; CHECK: mov    disp, disp+2    ; encoding: [0x90,0x40,A,A,B,B]
 ; CHECK:                        ;   fixup A - offset: 2, value: disp, kind: fixup_16_pcrel_byte
 ; CHECK:                        ;   fixup B - offset: 4, value: disp+2, kind: fixup_16_pcrel_byte
 
          jmp    foo
 ; CHECK: jmp    foo             ; encoding: [A,0b001111AA]
 ; CHECK:                        ;   fixup A - offset: 0, value: foo, kind: fixup_10_pcrel
+
+; RUN: llvm-mc -filetype=obj -triple msp430 < %s | llvm-readobj -r \
+; RUN:   | FileCheck -check-prefix=RELOC %s
+.short  _ctype+3
+; RELOC: R_MSP430_16_BYTE _ctype 0x3