musl: config is broken for !EXPERIMENTAL

If EXPERIMENTAL is not set, the only choice for version is the set of
released versions - currently, 1.1.14. But this only option is disabled
because it is also marked EXPERIMENTAL; this leaves no available choices
in the configuration.

Marking MUSL as experimental: it seems to have header issues which
prevent, for example, gdbserver from building. musl copied chunks of
ptrace.h code from the kernel into its own headers, which now clash with
Linux kernel headers. Manifests at least on SH4 target.

Also, musl breaks in powerpc builds: GCC balks at it with "unsupported
DEFAULT_LIBC" message. Also, 64-bit powerpc and mips are not supported.
So, until someone figures out the dependencies for musl in config/, mark
it experimental.

Signed-off-by: Alexey Neyman <stilor@att.net>

config/libc/musl.in

@@ -1,6 +1,7 @@
 # musl options
 
 ## depends on ! WINDOWS && ! BARE_METAL
+## depends on EXPERIMENTAL
 ##
 ## select LIBC_SUPPORT_THREADS_NATIVE
 ## select CC_CORE_PASSES_NEEDED