Disable PHP config option "pcre.jit" to work around PCRE segfault after "fork()"

Summary:
Ref T13296. On macOS with PHP 7.3.x, PCRE can segfault during calls to "preg_match()" or during process shutdown.

Disabling "pcre.jit" appears to work around this issue. Some casual Googling suggests that the JIT does not dramatically improve performance (~10%?) and we aren't PCRE-bound anyway, so prefer "not crashing" over "questionable performance improvements" on all systems for now.

Test Plan:

• See T13296 for details.
• Ran bin/phd start on macOS with PHP 7.3.1.
• Before patch: daemons don't actually start.
• After patch: daemons start.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13296

Differential Revision: https://secure.phabricator.com/D20554