Since clang 3.5.0 and later must be built by a compiler with C++11
support, make this explicit in src.opts.mk, by updating the default
settings.

The defaults become as follows:

• If the host compiler is not C++11 capable, use gcc and disable clang.
• On x86, enable clang, make it the default cc, and disable gcc.
• On little-endian ARM, enable clang, but not the full build, make it the default cc, and disable gcc.
• On PowerPC, enable clang, but enable gcc and make that the default cc.
• On everything else, use gcc, and disable clang.

This can be amended later, if we get e.g. sparc64 or big-endian ARM
working with clang.

Reviewed by: imp, brooks
Differential Revision: https://reviews.freebsd.org/D1294