s390/entry: remove toolchain dependent micro-optimization

Get rid of CONFIG_AS_IS_LLVM in entry.S to make the code a bit more
readable. This removes a micro-optimization, but given that the llvm IAS
limitation will likely stay, just use the version that works with llvm.

See commit 4c25f0ff

 ("s390/entry: workaround llvm's IAS limitations")
for further details.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>