Override the library search directories for external GCC toolchains.

[jhb]

Summary

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.

Test Plan

• was able to do a hosted buildworld using base/gcc in a FreeBSD/mips qemu machine across a revision that added a symbol to libnv and was failing when the /usr/lib fallback kicked in since it was trying to link to the wrong (old) libnv
• still able to use amd64-gcc with the CROSS_DIRECTORY_STRUCTURE hack removed. buildworld doesn't finish due to other breakages that have been present in HEAD for many months

[jhb]

Adding bdrewery@ and bapt@ in hope of getting some review. Not sure if there is another person who would be qualified to review GCC patches.

[kan]

I wonder what this does to, say, arm-none-eabi-gcc? It, in somewhat misguided manner, uses powerpc64-gcc as the base.

[jhb]

In D18677#402462, @kan wrote:

I wonder what this does to, say, arm-none-eabi-gcc? It, in somewhat misguided manner, uses powerpc64-gcc as the base.

I believe it should not be applied since arm-none (and the aarch64-none) define GCC_TARGET and I only add this to EXTRA_PATCHES when GCC_TARGET isn't defined.

[kan]

fair enough, I cannot see any negatives to this at the moment.