Override the library search directories for external GCC toolchains.

The default library search directories when building a native GCC end
up falling back to /usr/local/lib (for devel/*-gcc) and /usr/lib (for
base/gcc) even when --sysroot is used. For devel/*-gcc, I previously
forced CROSS_DIRECTORY_STRUCTURE on in r466699 to workaround this.
However, this solution could not be used for base/gcc. Instead, patch
the sources to remove several of the library search directories and
override STARTFILE_PREFIX_SPEC to only look in /usr/lib. GCC still
adds tuple-specific subdirs to /usr/lib, but this is ok.

Bump PORTREVISION for both the external GCC toolchains and base/gcc.

Reviewed by: kan
Differential Revision: https://reviews.freebsd.org/D18677