python3: Do not detect multiarch when cross compiling

python3: Do not detect multiarch when cross compiling

This was a long standing problem seen on aarch64 build hosts when
compiling python3 with clang cross compiler. The issue is not seen with
gcc because native glibc headers are still compatible with gcc cross compiler

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Ross Burton <ross.burton@arm.com>
Cc: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>