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

[dim]

Summary

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.

[rpokala]

I confirmed that this mips.mips.buildworld works; the other MIPS builds, PowerPC, and Sparc64 are in progress.

[rpokala]

make tinderbox JFLAG=-j16 TARGETS="mips powerpc sparc64" UNIVERSE_TARGET=buildworld

Ran to completion and passed.

[emaste]

Makefile.inc1

627 ↗

(On Diff #31328)

This XXX comment can probably go now.

[bdrewery]

The same logic is in Makefile.libcompat and may need to be handled as well.