Bring over some fixes from the other external GCC toolchain ports.

This is generally inspired by the changes in r465416 and subsequent fixes.

• Remove include-fixed headers.
• Strip /usr/local/include from the default include path.
• Use libc++ for C++ instead of libstdc++.
• Force build of a cross compiler if built on a riscv64 host.
• Bump PORTREVISION.

Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D14787