base/*: Remove the base ports to install an external toolchain in /usr.

[jhb]

Summary

This was originally motivated for providing a native toolchain for
platforms not supported by the in-tree LLVM toolchain. Now, however,
all of our current platforms are well supported by the in-tree LLVM
toolchain. In addition, these ports required additional maintenance
beyond the existing devel/binutils and devel/freebsd-gccX ports. A
more sustainable path if we need this in the future is probably to
have simple wrappers for /usr/bin/cc and other binaries in /usr that
redirect to binaries provided by the existing devel/ ports.

[jhb]

I know there is also some stuff in ports/Mk for things like CROSS_TOOLCHAIN, etc. That is potentially useful for other cross-building setups though, but I'm not sure if there any other base/ specific bits in other parts of the tree? (Also, if we do want to remove CROSS_TOOLCHAIN and friends from Mk/, I'd rather @bapt or someone else more experienced in Mk/ handle that.)

[brooks]

In D38561#877328, @jhb wrote:

I know there is also some stuff in ports/Mk for things like CROSS_TOOLCHAIN, etc. That is potentially useful for other cross-building setups though, but I'm not sure if there any other base/ specific bits in other parts of the tree? (Also, if we do want to remove CROSS_TOOLCHAIN and friends from Mk/, I'd rather @bapt or someone else more experienced in Mk/ handle that.)

I'd like to preserve some cross building support so we have a way to cross bootstrap llvm packages. D29236 was a stab at making CROSS_TOOCHAIN work for cmake (and seems to have been sufficient for LLVM with a a couple years ago with a CMAKE_TOOLCHAIN_FILE).