riscv: Cache the max alignment of output sections

riscv: Cache the max alignment of output sections

Cache the max alignment of output sections instead of scanning all
output sections for each input section, which can take a very long
time if there are millions of input/output sections.

	PR ld/22274
	* elfnn-riscv.c (riscv_elf_link_hash_table): Add max_alignment.
	(riscv_elf_link_hash_table_create): Initialize max_alignment to
	(bfd_vma) -1.
	(_bfd_riscv_relax_section): Cache the max alignment of output
	sections if possible.