add support for mips(el)-unknown-linux-musl

This target covers MIPS devices that run the trunk version of OpenWRT.

The x86_64-unknown-linux-musl target always links statically to C libraries. For
the mips(el)-unknown-linux-musl target, we opt for dynamic linking (like most of
other targets do) to keep binary size down.

As for the C compiler flags used in the build system, we use the same flags used
for the mips(el)-unknown-linux-gnu target.

configure

@@ -1178,7 +1178,7 @@ do
             ;;
 
 
-        *-musl)
+        x86_64-*-musl)
             if [ ! -f $CFG_MUSL_ROOT/lib/libc.a ]
             then
                 err "musl libc $CFG_MUSL_ROOT/lib/libc.a not found"