Strip undefined symbols from .symtab
bfd/
PR ld/4317
* elflink.c (elf_link_input_bfd): Drop undefined local syms.
(elf_link_output_extsym): Drop local and global undefined syms.
Tidy. Expand comment.
ld/testsuite/
PR ld/4317
* ld-aarch64/gc-tls-relocs.d, * ld-cris/locref2.d,
* ld-elf/ehdr_start-weak.d, * ld-elf/group1.d,
* ld-i386/compressed1.d, * ld-ia64/error1.d, * ld-ia64/error2.d,
* ld-ia64/error3.d, * ld-mips-elf/pic-and-nonpic-1.nd,
* ld-mmix/undef-3.d, * ld-powerpc/tlsexe.r, * ld-powerpc/tlsexetoc.r,
* ld-powerpc/tlsso.r, * ld-powerpc/tlstocso.r,
* ld-x86-64/compressed1.d, * ld-x86-64/pie1.d: Update.
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