builtins test: Move clear_cache_test.c from a mprotect()ed global to a mmap()ed variable

ld64 in the macOS 10.15 SDK gives __DATA a maxprot of 3, meaning it
can't be made executable at runtime by default.

Change clear_cache_test.c to use mmap()ed data that's mapped as writable
and executable from the beginning, instead of trying to mprotect()ing a
__DATA variable as executable. This fixes the test on macOS with the
10.15 SDK.

PR43407.

Differential Revision: https://reviews.llvm.org/D67929

llvm-svn: 372849