s390: move _text to an actual .text start

Since the uncompressed image .text section starts at 0x100000 now there
is no need to redefine _text to something else to make perf happy.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>