meson: Force use of LLVM ORCJIT for hosts without MCJIT support

Although the ORCJIT codepath is fresh and relatively less tested, this
is still better than no llvmpipe at all for those newer architectures
that will not gain MCJIT support, such as LoongArch or RISC-V.

Fixes: 6f02ec5ed1 ("llvmpipe: add an implementation with llvm orcjit")
Reviewed-by: Icenowy Zheng <uwu@icenowy.me>
Reviewed-by: Yukari Chiba <i@0x7f.cc>
Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30599>

meson_options.txt

@@ -444,7 +444,10 @@ option (
   'llvm-orcjit',
   type : 'boolean',
   value : false,
-  description: 'Build llvmpipe with LLVM ORCJIT support.'
+  description: 'Build llvmpipe with LLVM ORCJIT support. Has no effect when ' +
+               'building for architectures without LLVM MCJIT support -- ' +
+               'ORCJIT is the only choice on such architectures and will ' +
+               'always be enabled.'
 )
 
 option(