Drop Solaris support for MJIT

I tried to debug:
http://rubyci.s3.amazonaws.com/solaris10-gcc/ruby-master/log/20220905T070005Z.fail.html.gz
but I don't have sudo privilege to install dependencies such as xz on
that machine. Thus I can't extract a prebuilt libclang binary.

Anyway, if we find out ABI is different from x86_64 / aarch64, we'd like
to have sparc CI associated to GitHub to run `make mjit-bindgen`, but we
can't. Supporting this could be too hard, so I'm leaving it for now.

configure.ac

@@ -3744,7 +3744,7 @@ AC_ARG_ENABLE(jit-support,
     AS_HELP_STRING([--disable-jit-support], [disable JIT features]),
     [MJIT_SUPPORT=$enableval],
     [AS_CASE(["$target_os"],
-        [wasi | mingw*], [MJIT_SUPPORT=no],
+        [wasi | mingw* | solaris*], [MJIT_SUPPORT=no],
         [MJIT_SUPPORT=yes]
     )])