www/php84-opcache: fix build on non-amd64 / non-i386

post-configure step unconditionally tried to use JIT source, even when JIT was
disabled in ports (on aarch64) or in the source (powerpc*, riscv*, armv7),
which caused build breakage.

This commit checks whether JIT is supported (currently it is only on aarch64,
amd64 and i386) and only then uses JIT sources.
It also enables JIT on aarch64, which was disabled previously.