Permit base/binutils to be built as a native package.
- Don't require CROSS_TOOLCHAIN and CROSS_SYSROOT for base/binutils.
- Fix some places in devel/binutils to test the PREFIX instead of CROSS_TOOLCHAIN to determine if base/binutils is being built.
- When base/binutils is built natively, the binaries are installed with the target name prefix already.
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D19128