libc++: perform configuration checks with -nodefaultlibs

We're compiling libc++ with -nodefaultlibs, so we should also pass this
option during the configuration checks to ensure those checks are
consistent with the actual build.

The primary motivation here is to ease cross-compilation against a
non-standard set of C++ libraries. Previously, the configuration checks
would attempt to link against the standard C++ libraries, which would
cause link failures when cross-compiling, even though the actual library
link would go through correctly (because of the use of -nodefaultlibs
and explicitly specifying any needed libraries). This is more correct
even ignoring the motivation, however.

Patch by Shoaib Meenai!

llvm-svn: 280015