[Clang] Force rtlib=platform in test to avoid fails with CLANG_DEFAULT_RTLIB

Driver test `cross-linux.c` fails when CLANG_DEFAULT_RTLIB is "compiler-rt"
as the it expects a GCC-style `"crtbegin.o"` after `"crti.o"` but instead
receives something akin to this in the frontend invocation:

```
"crt1.o" "crti.o"
"/o/b/llvm/bin/../lib/clang/10.0.0/lib/linux/clang_rt.crtbegin-x86_64.o"
```

This patch adds an override to `cross-linux.c` tests so the expected result
is produced regardless of the compile-time default rtlib, as having tests
fail due to that is fairly confusing. After applying the patch, the test
passes regardless of the CLANG_DEFAULT_RTLIB setting.

Differential Revision: https://reviews.llvm.org/D72236