Allow for __gnu_lto_slim prefixed with extra "_"

Allow for __gnu_lto_slim prefixed with extra "_"

Some targets prefix global symbols with "_".

bfd/
	* archive.c (_bfd_compute_and_write_armap): Match "__gnu_lto_slim"
	optionally prefixed with "_".
	* linker.c (_bfd_generic_link_add_one_symbol): Likewise.
binutils/
	* nm.c (filter_symbols): Match "__gnu_lto_slim" optionally prefixed
	with "_".
gold/
	* symtab.cc (Symbol_table::add_from_relobj): Match "__gnu_lto_slim"
	optionally prefixed with "_".
ld/
	* testsuite/ld-plugin/lto-3r.d: Match "__gnu_lto_v" optionally
	prefixed with "_".
	* testsuite/ld-plugin/lto-5r.d: Likewise.