Fix combination of new-smt-backend and gcc-only test tags

Tests which were tagged with both `new-smt-backend` and `gcc-only` would
previously be executed on Windows with Visual Studio test jobs. The
`gcc-only` tag is used when tests cover features which are not supported
by Visual Studio. Therefore running them with Visual Studio is a an
error on the part of the test framework, rather than representative of
an issue with cbmc. Propagating `gcc_only_string` to the addition of the
`new-smt-backend` test profile will switch off running these tests for
the Visual Studio platform only.

Author: thomasspriggs

regression/cbmc/CMakeLists.txt

@@ -34,7 +34,7 @@ add_test_pl_profile(
 add_test_pl_profile(
   "cbmc-new-smt-backend"
   "$<TARGET_FILE:cbmc> --incremental-smt2-solver 'z3 --smt2 -in'"
-  "-I;new-smt-backend;-s;new-smt-backend"
+  "${gcc_only_string}-I;new-smt-backend;-s;new-smt-backend"
   "CORE"
 )