Fix forced allocation of common (-d) during -r links.

Fix forced allocation of common (-d) during -r links.

If the .bss section has other data in it besides common allocations,
gold was subtracting the wrong section start address from the symbol
value.

gold/
	PR gold/20976
	* symtab.cc (Symbol_table::sized_write_globals): Use address of
	output section, not input section.
	* testsuite/Makefile.am (pr20976): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/pr20976.c: New source file.