Fix alpha-elf relaxation

ld/
	* emultempl/alphaelf.em (alpha_after_parse): Enable 2 relax passes.
bfd/
	* elf64-alpha.c (elf64_alpha_size_got_sections): New may_merge
	parameter; honor it and disable got merging when false.
	(elf64_alpha_relax_got_load): Do not relax to GPREL relocs during
	the first pass of relaxation.
	(elf64_alpha_relax_with_lituse): Likewise.  Move relaxed relocs to
	the end of the LITERAL+LITUSE chain.
	(elf64_alpha_relax_section): Only process LITERAL relocs during the
	second pass of relaxation.