[builtins][test] Avoid unportable mmap call in clear_cache_test.c

[builtins][test] Avoid unportable mmap call in clear_cache_test.c

Within the last two weeks, the Builtins-*-sunos :: clear_cache_test.c started to FAIL
on Solaris.  Running it under truss shows

  mmap(0x00000000, 128, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANON, 0, 0) Err#22 EINVAL
  _exit(1)

While there are several possible reasons mmap can return EINVAL on Solaris, it turns
out it's this one (from mmap(2)):

  MAP_ANON  was  specified,  but the file descriptor was not
   -1.

And indeed even the Linux mmap(2) documents this as unportable:

  MAP_ANONYMOUS
          The mapping is not backed by any file; its contents are initial‐
          ized to zero.  The fd argument is ignored; however, some  imple‐
          mentations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is
          specified, and portable applications should  ensure  this.   The

This patch follows this advise.  Tested on x86_64-pc-linux-gnu, amd64-pc-solaris2.11
and sparcv9-sun-solaris2.11.

Differential Revision: https://reviews.llvm.org/D68455

llvm-svn: 375490