elf: Use mmap to map in read-only sections

There are many linker input files in LLVM debug build with huge string
sections.  All these string sections can be treated as read-only.  But
linker copies all of them into memory which consumes huge amount of
memory and slows down linker significantly.

Add _bfd_mmap_readonly_persistent and _bfd_mmap_readonly_temporary to
mmap in reado-only sections with size >= 4 * page size.

NB: All string sections in valid ELF inputs must be null terminated.
There is no need to terminate it again and string sections are mmapped
as read-only.

	* bfd.c (bfd_mmapped_entry): New.
	(bfd_mmapped): Likewise.
	(bfd): Add mmapped.
	* bfdwin.c (bfd_get_file_window): Use _bfd_pagesize.
	* cache.c (cache_bmmap): Remove pagesize_m1 and use pagesize_m1
	instead.
	* elf.c (bfd_elf_get_str_section): Call
	_bfd_mmap_readonly_persistent instead of _bfd_alloc_and_read.
	Don't terminate the string section again.
	(get_hash_table_data): Call _bfd_mmap_readonly_temporary and
	_bfd_munmap_readonly_temporary instead of _bfd_malloc_and_read
	and free.
	(_bfd_elf_get_dynamic_symbols): Call _bfd_mmap_readonly_persistent
	instead of _bfd_alloc_and_read.  Don't terminate the string
	section again.  Call _bfd_mmap_readonly_temporary and
	_bfd_munmap_readonly_temporary instead of _bfd_malloc_and_read
	and free.
	(_bfd_elf_slurp_version_tables): Call _bfd_mmap_readonly_temporary
	and _bfd_munmap_readonly_temporary instead of _bfd_malloc_and_read
	and free.
	* elflink.c (bfd_elf_link_record_dynamic_symbol): Use bfd_malloc
	to get the unversioned symbol.
	* libbfd-in.h (_bfd_pagesize): New.
	(_bfd_pagesize_m1): Likewise.
	(_bfd_minimum_mmap_size): Likewise.
	(_bfd_mmap_readonly_persistent): Likewise.
	(_bfd_mmap_readonly_temporary): Likewise.
	(_bfd_munmap_readonly_temporary): Likewise.
	* libbfd.c
	(bfd_allocate_mmapped_page): New.
	(_bfd_mmap_readonly_temporary): Likewise.
	(_bfd_munmap_readonly_temporary): Likewise.
	(_bfd_mmap_readonly_persistent): Likewise.
	(_bfd_pagesize): Likewise.
	(_bfd_pagesize_m1): Likewise.
	(_bfd_minimum_mmap_size): Likewise.
	(bfd_init_pagesize): Likewise.
	* lynx-core.c (lynx_core_file_p): Use _bfd_pagesize.
	* opncls.c (_bfd_delete_bfd): Munmap tracked mmapped memories.
	* sysdep.h (MAP_ANONYMOUS): New. Define if undefined.
	* bfd-in2.h: Regenerated.
	* libbfd.h: Likewise.