Each of the submodule configuration files defined a subset of the
cross compiler tools that it used and many were picking up the
system `ar`, `nm`, `strip, `ld`, etc.  They all now use a `Makefile`
macro that defines the path to the proper cross compiler tools.

For ones that need the tools, but not the musl-libc gcc,
there is $(CROSS_TOOLS_NOCC) that is all of them without gcc.
This is for musl-libc itself, as well as xen and the Linux kernel.