Summary:
Convert from jemalloc's obsolete *allocm() to *allocx().

Strengthen goodMallocSize() to always return a jemalloc size class, so
that xallocx() success/failure detection is simple.

@override-unit-failures

Test Plan: Folly, unicorn, and HHVM tests.

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: trunkagent, hphp-diffs@, ps, chaoyc, search-fbcode-diffs@, unicorn-diffs@, ptc, njormrod

FB internal diff: D1535841

Tasks: 4996808