temporary get rid of ifunc in folly::Range

temporary get rid of ifunc in folly::Range

Summary:
See discussion in D638500.

valgrind 3.7 contains a bug that makes it hung up
(https://bugs.kde.org/show_bug.cgi?id=301204).

glibc 2.5.1 doesn't support ifunc.

In the future we should conditionally compile ifunc stuff (this is also
required to support clang).

Test Plan:
unittests, ran range_benchmark to confirm there is no
significant performance drop, this additional level of indirection adds overhead of ~1ns.

tested on CPUs with and without SSE4.2 support.

Reviewed By: mmcurtiss@fb.com

FB internal diff: D687351