[siddiqueirshad]

Summary of changes

On FreeBSD:

1. When \FOLLY_USE_JEMALLOC\ is \OFF, \olly/portability/Malloc.h\ did not include <malloc_np.h>, which resulted in missing declarations for \mallocx,
   allocx, \mallctl, and related allocator APIs in downstream translation units. This change includes <malloc_np.h>\ on FreeBSD in the fallback branch of \olly/portability/Malloc.h\ so allocator symbols remain available.
2. In \olly/memory/test/MallctlHelperTest.cpp, FreeBSD supplies non-standard allocator declarations in <malloc_np.h>\ rather than <jemalloc/jemalloc.h>. Conditionally include <malloc_np.h>\ on FreeBSD and safely guard the check on \JEMALLOC_VERSION_MAJOR.

Closes #2720
Closes #2721
Closes #1893

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