x86: Cache the symbol table when packing relative relocations

When packing relative relocations, x86 linker may load the same symbol
table repeatedly, which can take a long time.  On Intel Core i7-1195G7
with 32GB RAM, it takes more than 45 minutes to create an output with
-pie -z pack-relative-relocs from an input with 208025 code sections.
Cache the symbol table to reduce the link time to less than 2 seconds.

On the same machine, creating 3.1GB clang executable in LLVM 21.1.3 debug
build:

user            55.39 seconds
system          6.71 seconds
total           65.80 seconds
maximum set(GB) 10.43
page faults     2406941

	PR ld/33765
	* elfxx-x86.c (elf_x86_relative_reloc_record_add): Remove
	keep_symbuf_p.
	(_bfd_x86_elf_link_relax_section): Updated.  Cache the symbol
	table to avoid loading it again.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>