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.