[LoongArch] Add some fixups and relocations

This patch only add %pc_hi20/%pc_lo12/%plt relocations in order
to be able to generate gnu ld linkable relocation file for the
`hello world` IR :
```
@.str = private unnamed_addr constant [14 x i8] c"Hello world!\0A\00", align 1

define dso_local signext i32 @main() nounwind {
entry:
  %call = call signext i32 (ptr, ...) @printf(ptr noundef @.str)
  ret i32 0
}

declare dso_local signext i32 @printf(ptr noundef, ...)
```

This patch also updates some test cases due to new modifiers introduced.
New test: test/MC/LoongArch/Relocations/relocations.s

Differential Revision: https://reviews.llvm.org/D132108