cmd/link: fix loadelf failed on MIPS family

cmd/link: fix loadelf failed on MIPS family

The relocation of MIPS64 family ELF is different with other architecure according
to the document from Linux-MIPS

https://www.linux-mips.org/pub/linux/mips/doc/ABI/elf64-2.4.pdf

In "2.9 Relocation" it shows relocation section contains five parts:

1. r_sym Elf64_Word Symbol index
2. r_ssym Elf64_Byte Special symbol
3. r_type3 Elf64_Byte Relocation type
4. r_type2 Elf64_Byte Relocation type
5. r_type Elf64_Byte Relocation type

This CL makes loadelf aware the difference.

Update #35779

Change-Id: Ib221665641972b1c2bfea5a496e3118e5dc0bc45
Reviewed-on: https://go-review.googlesource.com/c/go/+/209317
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>