[libcxx] Fix include directory order (#65859)

It's important that the arch directory be included first so that
its header files which interpose on the default include dir
be included instead of the default ones. The clang driver [1] does
this when not building with -nostdinc, the libcxx build should
do the same.

We found this after https://reviews.llvm.org/D154282 when cross
compiling from non Linux to Linux. If the host machine was not
Linux, _LIBCPP_HAS_NO_TIME_ZONE_DATABASE would be defined in
the default include dir __config_site, while it was undefined
in the arch specific one causing build failures.