s390: put _stext and _etext into .text section

Perf relies on _etext and _stext symbols being one of 't', 'T', 'v' or
'V'. Put them into .text section to guarantee that.

Also moves padding to page boundary inside .text which has an effect that
.text section is now padded with nops rather than 0's, which apparently
has been the initial intention for specifying 0x0700 fill expression.

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Suggested-by: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>