Per https://gcc.gnu.org/onlinedocs/gcc/PowerPC-Hardware-Transactional-Memory-Built-in-Functions.html

The .. high level HTM interface .. is common between PowerPC and S/390

Reimplemented the transactional_lock_enabled() detection mechanism for
s390x and POWER based on SIGILL. This also gives non-Linux based unixes
the ability to use HTM. The implementation is based off openssl.
(ref:
https://github.com/openssl/openssl/blob/1c0eede9827b0962f1d752fa4ab5d436fa039da4/crypto/s390xcap.c#L104)

The other ppc64{,le} problems with getauxvec based detection:
* Checking PPC_FEATURE2_HTM_NOSC not needed as we do not do syscalls while
  in a transactional state.
* As we don't use, and never should use PPC_FEATURE2_HTM_NO_SUSPEND,
  or do syscalls while in transactional state, don't test it.
From: https://www.kernel.org/doc/html/v5.4/powerpc/syscall64-abi.html#transactional-memory

S390x high level __builtin_tbegin functions in the htmxlintrin.h are not
inline. This header file can be included once in the entire set of sources for
a linked target, otherwise duplicate symbols occur. While we could use inline
xabort/xend functions using the low level interface, we keep this the same as
ppc64 for simplicity.

SLES-15, gcc-7, appeared to want everything that included the htmlxlintrin to
be compiled with -mhtm otherwise the __builtin_t{func} where not defined
(in addition to a #ifdef __HTM__ #error). Debian sid gcc-11.2 wanted the same
on ppc64le/ppc64. In general we want to avoid a wide spread use of architecture
cflags as it makes justifications for selective optimizations easier.
(ref: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1006702)