[J-core] musl-cross-make can now install kernel headers

[Rich Felker]

I've updated the musl-cross-make toolchain build system to support
fetching kernel sources and installing the kernel headers for the
target. This should make it easier to use the resulting toolchains
for building things like toybox, busybox, strace, etc. that need
kernel headers, and for using them with buildroot. As usual the repo
is here:

https://github.com/richfelker/musl-cross-make

The kernel header functionality can be suppressed by clearing the
LINUX_VER make variable, if desired.

The new version also installs a $(TARGET)-cc symlink for gcc, for
compatibility with software that only tries cc and not gcc.

Rich