Change the way JIT availability is checked

Change the way JIT availability is checked

The pcre2_jit_compile_8 sysmbol is always available, even JIT might be
not. If JIT is not enabled explicitly and is enabled in the PHP runtime,
this will lead to a malfunction. This approach ensures JIT is indeed
available on the given platform. For cross compilation this might get
complicated, as it would require an explicit processor architecture and
PCRE2 version check.

Another solution for this case is to run pcre2_config at runtime. That
however would require more condition checks that would impact
architectures where JIT is available.