Clear SHF_COMPRESSED flag bit from input to output

Clear SHF_COMPRESSED flag bit from input to output

For relocatable link, we should clear the SHF_COMPRESSED flag bit from
input group section.

	PR gold/18689
	* layout.cc (Layout::layout): Clear the SHF_COMPRESSED flag bit
	from input group section for relocatable link.
	* testsuite/Makefile.am (check_SCRIPTS): Add pr18689.sh.
	(check_DATA): Add pr18689.stdout.
	(MOSTLYCLEANFILES): Add pr18689a.o pr18689b.o.
	(pr18689.stdout): New rule.
	(pr18689a.o): Likewise.
	(pr18689b.o): Likewise.
	(pr18689.o): Likewise.
	* testsuite/pr18689.c: New file.
	* testsuite/pr18689.sh: Likewise.
	* testsuite/Makefile.in: Regenerated.