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