binutils: build as with BINUTILS || BINUTILS_BOOTSTRAP

Previously we descended into as only if MK_BINUTILS was true, including
during the bootstrap tool phase. BINUTILS is now disabled by default on
all archs, and we failed to build it during amd64 bootstrap.

Descend into as if either BINUTILS or BINUTILS_BOOTSTRAP is enabled.

This is not quite correct: we should either have the test also depend on
BOOTSTRAPPING, or set BINUTILS to the value of BINUTILS_BOOTSTRAP during
the bootstrap phase. However, this simple change fixes the build and
has been tested, and binutils will be removed completely in the near
future.