Default to disabling the libunwind frameheader cache.

Although it works fine with glibc, as currently implemented the
frameheader cache is incompatible with certain platforms with
slightly different locking semantics inside dl_iterate_phdr.

Therefore only enable it when it is turned on explicitly with
a configure-time option.

Differential Revision: https://reviews.llvm.org/D86163

(cherry picked from commit a20f5fe7)