This commit prepares the build script for a wasm32 target that doesn't use
Emcripten, notably forcing the `mem` feature to get activated and forcibly
ignoring the `c` feature, even if activated, for the wasm32 target.
* Don't run `intrinsics` tests on thumb
* Disable `compiler_builtins` attribute on `feature = "gen-tests"`
* Disable mangling on `feature = "gen-tests"` instead of `cfg(test)`
This is an attempt to tidy up the definition of intrinsics by making them more
rust-like at the definition site and using traits instead of macros for the
definition. Additionally the helper macro, `intrinsics!`, now fills in a
definition for #[cfg]'d off intrinsics when compiling with C code
