[asan] Clean up the REQUIRES of `asan/TestCases/strcat-overlap.cc`

by explicitly listing where we expect this test to work.

Because this test invokes undefined behaviour all sorts of things
can happen (e.g. crash, or on some bots asan manages to catch
something). The test seems to pass okay on Darwin (x86_64/x86_64/i386)
and on Linux (x86_64/i386) so explicitly require one of these platforms
to run the test.

llvm-svn: 327185