s390: move expoline assembler macros to a header

To be able to use the expoline branches in different assembler
files move the associated macros from entry.S to a new header
nospec-insn.h.

While we are at it make the macros a bit nicer to use.

Cc: stable@vger.kernel.org # 4.16
Fixes: f19fbd5e

 ("s390: introduce execute-trampolines for branches")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>