interp: Simplify value conversion

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Fix clang build when LLVM is enabled

We don't use LLVM's default CXXFLAGS because they disable exceptions.
Clang requires __STDC_LIMIT_MACROS to be defined so fix that up.

Signed-off-by: Pekka Enberg <penberg@kernel.org>