test: Disable standard system includes in %clang_cc1

This adds -nostdsysteminc to the %clang_cc1 expansion, which should
make it harder to accidentally write tests that depend on headers in
/usr/include. It also updates a few tests that use -isysroot <x> and a
darwin triple to omit the triple and use -isystem <x>/usr/include
instead, making them a little bit more general.

Incidentally, this fixes a test failure I'm seeing on darwin in
Modules/stddef.c, that happens because my system finds a stddef.h in
/usr/include.