ruby: Link with libucontext on musl

ruby: Link with libucontext on musl

coroutines in ruby 2.7+ needs ucontext APIs which are not available in
musl but an external library is available to provide them so use it

Use cached values for ac_cv_func_isnan and ac_cv_func_isinf this is not
detected correctly by configure on musl

on ARM drop using old arm32 implementation of coroutine which is slow and
inefficient

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>