Skip separate debug files when handling copy relocations

get_symbol_address and get_msymbol_address call
lookup_minimal_symbol_linkage, which iterates over the separate debug
files of the objfile that is passed in.

This means that if these functions pass in a separate debug objfile,
then they are doing unnecessary work.

This patch avoids the extra work by skipping separate debug objfiles
in the loops.

gdb/ChangeLog
2020-04-10  Tom Tromey  <tromey@adacore.com>

	* symtab.c (get_symbol_address, get_msymbol_address): Skip
	separate debug files.