MFC r321684:

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
Differential Revision: https://reviews.freebsd.org/D11776

MFC r322170:

Follow-up to r321684 (Don't use libc++ when cross-building for gcc
arches), and handle two more cases where libc++ includes could be
incorrectly enabled, in case the host compiler is clang 5.0.0, and the
target (cross) compiler is gcc 4.2.1.

Noted by: bdrewery