debug/elf,cmd/link: add additional MIPS64 relocation type

debug/elf,cmd/link: add additional MIPS64 relocation type

Add R_MIPS_PC32 which is a 32 bit PC relative relocation.

These are produced by LLVM on mips64.

Fixes #61974

Change-Id: I7b6c6848e40249e6d5ea474ea53c9d7e3ab23f88
Reviewed-on: https://go-review.googlesource.com/c/go/+/469395
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Junxian Zhu <zhujunxian@oss.cipunited.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>