Per https://gcc.gnu.org/onlinedocs/gcc/PowerPC-Hardware-Transactional-Memory-Built-in-Functions.html, the builtins are only available when -mhtm or -mcpu=power8 or later. Neither of these are used in the compile.
MDEV-26769-htm-flag-for-ppc64 pushed, see how buildbots handle it. Proper fix might be to make the hardware lock ellision somehow dependent on compile flag detection. I'm also unsure if AIX supports it.
HTM was introduced POWER8, so I'm unsure if older POWER will survive this change.