elf/elf.h: Add new 386 and X86_64 relocations from binutils.

The following new 386 and X86_64 were added to binutils. They are
non-dynamic relocations, so don't need direct handling in glibc.
But other programs, like elfutils, use the glibc elf.h definitions
for the names and numbers when inspecting ET_REL files.

R_386_GOT32X was proposed in
https://groups.google.com/forum/#!topic/ia32-abi/GbJJskkid4I

X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX were proposed in
https://groups.google.com/forum/#!topic/x86-64-abi/n9AWHogmVY0

There also used to be R_X86_64_PC32_BND and R_X86_64_PLT32_BND
but those already got deprecated in
https://groups.google.com/d/msg/x86-64-abi/-hdQyMixt8Y/XFDOvioG85cJ

	* elf/elf.h (R_386_GOT32X): New.
	(R_386_NUM): Update.
	(R_X86_64_GOTPCRELX: New.
	(R_X86_64_REX_GOTPCRELX): New.
	(R_X86_64_NUM): Update.