intel/compiler: Convert brw_eu.cpp back to brw_eu.c

Now that we've removed the thread_local lookup tables using
pointer-to-member C++ features, this can go back to being a standard
C file, like it was in the past.  We just need to annotate a couple
of things with "struct".

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17309>

src/intel/compiler/meson.build

@@ -38,7 +38,7 @@ libintel_compiler_files = files(
   'brw_disasm.c',
   'brw_disasm_info.c',
   'brw_disasm_info.h',
-  'brw_eu.cpp',
+  'brw_eu.c',
   'brw_eu_compact.c',
   'brw_eu_defines.h',
   'brw_eu_emit.c',