Don't disable binutils/elftoolchain bootstrapping with external compiler.

This was a regression from r300349.

Setting MK_CROSS_COMPILER=no forces the compiler bootstraping *and* the
binutils/elftoolchain bootstrapping to be disabled in share/mk/src.opts.mk.
The only intent with using an external compiler is to disable bootstrapping of
the compiler. The binutils/elftoolchain bootstrapping must still occur unless
XAS is set. This did not affect WITH_SYSTEM_COMPILER.

Now that setting an external compiler sets both MK_CLANG_BOOTSTRAP and
MK_GCC_BOOTSTRAP to no, and MK_CROSS_COMPILER does the same via
share/mk/src/opts.mk, remove redundant logic that checks for
MK_CROSS_COMPILER. It will not always be true now that MK_CROSS_COMPILER==no
when an external compiler is used and --sysroot/-target is needed.

Reported by: sbruno
Pointyhat to: bdrewery
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division