[MC] Support constant offset for symbol PendingFixup

This patch add support relocation offset of sym+constant(like `foo+4`) form for pending fixup.

In the past, llvm-mc ignored the constant in sym+constant form, for `foo+4`, `4` would be ignored. And test case
```
.text
  ret
  nop
  nop
  .reloc foo+4, R_RISCV_32, 6

.data
.globl foo
foo:
  .word 0
  .word 0
  .word 0
```
when run `llvm-mc -filetype=obj -triple=riscv64 %s | llvm-readobj -r`
The output is
```
Relocations [
  Section (3) .rela.text {
    0x0 R_RISCV_32 - 0x6
  }
]
```

After applying this patch, the output is
```
Relocations [
  Section (3) .rela.text {
    0x4 R_RISCV_32 - 0x6
  }
]
```

Differential Revision: https://reviews.llvm.org/D117316