s390: Fix linker s390 emulation option parsing

Append s390-specific emulation options to the shell variables instead of
replacing their contents.

ld/
	* emultempl/s390.em (PARSE_AND_LIST_LONGOPTS,
	PARSE_AND_LIST_OPTIONS, PARSE_AND_LIST_ARGS_CASES): Append to
	emulation options instead of replacing them.

Fixes: b4cbbe8f7294 ("S/390: Add support for pgste marker")
Signed-off-by: Jens Remus <jremus@linux.ibm.com>