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.
