rustbuild incorrectly builds std for musl targets

[japaric]

It copies the crt*.o objects from the host system libraries when it, instead, should copy them from the musl-root. Related code

It asks rustc to pass -ldl -lpthread and -lrt to the linker when it shouldn't because those are already included in musl libc.a. Related code

cc @alexcrichton

[alexcrichton]

Hm the crt.o files look like they're coming from the target compiler, but maybe the target passed in is wrong? The -ldl and -lpthread stuff should definitely be omitted on musl!

[japaric]

$ gcc -print-file-name=crti.o
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crti.o
$ musl-gcc -print-file-name=crti.o
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crti.o

But the musl-root in my case is /musl. IOW, the musl crti.o is in /musl/lib/crti.o.

[alexcrichton]

Oh weird, I was hoping musl-gcc would do the right thing :(