s390/facility: Let test_facility() generate static branch if possible

s390/facility: Let test_facility() generate static branch if possible

Let test_facility() generate a branch instruction if the tested facility is
a constant, and where the result cannot be evaluated during compile
time. The branch instruction defaults to "false" and is patched to nop
(branch not taken) if the tested facility is available.

This avoids runtime checks and is similar to x86's static_cpu_has() and
arm64's alternative_has_cap_likely().

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>