elf: Use mmap to map in symbol and relocation tables

elf: Use mmap to map in symbol and relocation tables

Add _bfd_mmap_read_temporary to mmap in symbol tables and relocations
whose sizes >= 4 * page size.  For the final link, allocate an external
relocation buffer of 4 * page size to avoid using mmap and munmap on
smaller relocation sections.  Since _bfd_mmap_read_temporary allocates
buffer as needed, its callers don't need to.

When mmap is used to map in all ELF sections, data to link the 3.5GB
clang executable in LLVM 17 debug build on Linux/x86-64 with 32GB RAM
is:

		stdio		mmap		improvement
user		84.79		85.27		-0.5%
system		10.95		9.09		17%
total		97.91		94.90		3%
page faults	4837944		4033778		17%

and data to link the 275M cc1plus executable in GCC 14 stage 1 build
is:

user		5.31		5.33		-0.4%
system		0.86		0.76		12%
total		6.19		6.13		1%
page faults	361273		322491		11%

	* elf.c (bfd_elf_get_elf_syms): Don't allocate buffer for external
	symbol table.  Replace bfd_read with _bfd_mmap_read_temporary.
	* elflink.c (elf_link_read_relocs_from_section): Add 2 arguments
	to return mmap memory address and size.
	(_bfd_elf_link_info_read_relocs): Don't allocate buffer for
	external relocation information.  Replace bfd_read with
	_bfd_mmap_read_temporary.
	(bfd_elf_final_link): Cache external relocations up to
	_bfd_minimum_mmap_size bytes when mmap is used.
	* libbfd.c (_bfd_mmap_read_temporary): New.
	* libbfd-in.h (_bfd_mmap_read_temporary): Likewise.
	* libbfd.h: Regenerated.