Avoid need for preprocessor calls with -m64

This isn't portable to non-x86 architectures.

Author: tautschnig

regression/cbmc-concurrency/memory_barrier2/msvc.desc

@@ -1,5 +1,5 @@
 CORE
-msvc.c
+msvc.i
 --mm tso --winx64
 ^EXIT=0$
 ^SIGNAL=0$

regression/cbmc-concurrency/memory_barrier2/msvc.c renamed to regression/cbmc-concurrency/memory_barrier2/msvc.i

@@ -1,5 +1,5 @@
 CORE
-msvc.c
+msvc.i
 --pointer-check --bounds-check --winx64
 ^EXIT=0$
 ^SIGNAL=0$