Automatic upgrade of CBMC from 6.3.1 to 6.4.0 (model-checking#3689)

Upgrade CBMC to its latest release.

Co-authored-by: qinheping <16714939+qinheping@users.noreply.github.com>
Co-authored-by: Michael Tautschnig <tautschn@amazon.com>

kani-dependencies

@@ -1,6 +1,6 @@
 CBMC_MAJOR="6"
-CBMC_MINOR="3"
-CBMC_VERSION="6.3.1"
+CBMC_MINOR="4"
+CBMC_VERSION="6.4.0"
 
 # If you update this version number, remember to bump it in `src/setup.rs` too
 CBMC_VIEWER_MAJOR="3"