[J-core] updated cross-compiler build system

[Rich Felker]

Hi everyone,

I've just pushed the new overhaul of musl-cross-make, my cross
compiler build system:

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

This should make it faster (single-stage), safer (https and hash
checking), simpler (built-in fetching of gmp & friends), and cleaner
to build and install cross compilers that can be used to target J-core
Linux. The sh2eb-linux-muslfdpic target profile in the sample
config.mak(.dist) file is all you need to select and use. As usual it
also should work for all other (non J-core) musl targets too.

I've tested it pretty extensively on two systems I use for builds, but
as this is a big change/overhaul there might be things that break. Let
me know how using it goes.

Rich