Don't use libc++ when cross-building for gcc arches

Since we imported clang 5.0.0, the version check in Makefile.inc1 which
checks whether to use libc++ fires even when the compiler for the target
architecture is gcc 4.2.1. This is because only X_COMPILER_VERSION is
checked. Also check X_COMPILER_TYPE, so it will only use libc++ when an
external gcc toolchain is used.

Reviewed by: emaste, rpokala
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D11776