The GNU binutils recently picked up support for new 386/amd64
relocations.  Add support for them in the Go linker when doing an
internal link.

The 386 relocation R_386_GOT32X was proposed in
https://groups.google.com/forum/#!topic/ia32-abi/GbJJskkid4I .  It can
be treated as identical to the R_386_GOT32 relocation.

The amd64 relocations R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX were
proposed in
https://groups.google.com/forum/#!topic/x86-64-abi/n9AWHogmVY0 .  They
can both be treated as identical to the R_X86_64_GOTPCREL relocation.

The purpose of the new relocations is to permit additional linker
relaxations in some cases.  We do not attempt to support those cases.

While we're at it, remove the unused and in some cases out of date
_COUNT names from ld/elf.go.

Fixes #13114.

Change-Id: I34ef07f6fcd00cdd2996038ecf46bb77a49e968b
Reviewed-on: https://go-review.googlesource.com/16529Reviewed-by: Minux Ma <minux@golang.org>