Support out-of-tree binutils with in-tree compiler

[emaste]

Summary

Right now CROSS_BINUTILS_PATH is honoured only when XCC is set, i.e. an out-of-tree compiler. Allow CROSS_BINUTILS_PATH to pass through to a -B option also when using the in-tree compiler.

I think this could be named just BINUTILS_PATH, but the existing CROSS_BINUTILS_PATH could as well. Since they have the same meaning I used the existing, if somewhat confusing, name.

[emaste]

This supports aarch64 builds by installing the aarch64-binutils port and adding CROSS_BINUTILS_PREFIX=/usr/local/aarch64-freebsd/bin/ to the make command line.

[bapt]

Looks ok to me

[imp]

looks good to me. I've reviewed this in the past as well...

[emaste]

Closed by commit rS279328 (authored by @emaste).