Musl was marked experimental in commit 08d91d41 ("musl: config is broken
for !EXPERIMENTAL"). Most of the reasoning for that change no longer
applies and as it's been about 8 years it's time to let musl loose on
the world. Drop the `depends on EXPERIMENTAL` and update the sample
configs for aarch64 and x86_64.
For powerpc64 the ABI needs to be elfv2. Enforce this via the powerpc
config. Add a sample configuration for powerpc-unknown-linux-musl.
Signed-off-by: Chris Packham <judge.packham@gmail.com>
