meson: Add back execmem option as a deprecated option

I've been betting support requests by people confused as to why their
builds broke because this option was removed. We can add the option back
with the deprecated flag set so that Meson will give a nice warning, but
builds will continue to work.

fixes: 1dd1147408

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23893>

meson_options.txt

@@ -468,6 +468,13 @@ option(
                 'with allow_execmem.'
 )
 
+option(
+  'execmem',
+  type : 'boolean',
+  deprecated : true,
+  description : 'Does nothing, left here for a while to avoid build breakages.',
+)
+
 option(
   'osmesa',
   type : 'boolean',