[LoongArch] Allow %pc_lo12 relocs in JIRL's immediate operand position

Currently, gcc-13 will generate such assembly when `-mcmodel=medium`,
which is ostensibly a dirty hack to allow bigger offsets for extern
function calls without having to add more reloc types. This is not the
best way to accomplish the original goal, but such usages will appear
soon and we have to support it anyway.

Example:

```c
extern int foo(int);

int bar(int x) {
    return foo(x + 123);
}
```

will produce the following (simplified) assembly when compiled with
`-O2 -mcmodel=medium`:

```
    .globl  bar
    .type   bar, @function
bar:
    .cfi_startproc
    addi.w  $r4,$r4,123
    pcalau12i   $r12,%pc_hi20(foo)
    jirl    $r0,$r12,%pc_lo12(foo)
    .cfi_endproc
```

Reviewed By: SixWeining, wangleiat, MaskRay, xry111

Differential Revision: https://reviews.llvm.org/D142278