[test] Disable sunrpc tests when rpc/xdr.h is missing

Disable tests requiring sunrpc when the relevant headers are missing.
In order to accommodate that, move the header check
from sanitizer_common to base-config-ix, and define the check result
as a global variable there.  Use it afterwards both for definition
needed by sanitizer_common, and to control 'sunrpc' test feature.

While at it, remove the append_have_file_definition macro that was used
only once, and no longer fits the split check-definition.

Bug report: https://github.com/google/sanitizers/issues/974

Differential Revision: https://reviews.llvm.org/D47819

llvm-svn: 351109