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

[ro]

Summary

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. Ok for trunk?

[delcypher]

LGTM.

compiler-rt/test/builtins/Unit/clear_cache_test.c

 }
 
 int main()
 {
     const int kSize = 128;
 #if !defined(_WIN32)
     uint8_t *execution_buffer = mmap(0, kSize,
                                      PROT_READ | PROT_WRITE | PROT_EXEC,
-                                     MAP_ANON | MAP_PRIVATE, 0, 0);
+                                     MAP_ANON | MAP_PRIVATE, -1, 0);
     if (execution_buffer == MAP_FAILED)
       return 1;
 #else
     HANDLE mapping = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,
                                        PAGE_EXECUTE_READWRITE, 0, kSize, NULL);
     if (mapping == NULL)
         return 1;