Fix genapply for cross-compilation & enable wasm32 register mapping
This patch fixes incorrectly built genapply when cross compiling (#24347 (closed)) by nuking all fragile CPP logic in it from the orbit. All target-specific info are now read from DerivedConstants.h
at runtime. Additionally, this patch re-enables full register mapping for wasm32, and we can now generate smaller & faster wasm modules that doesn't always spill arguments onto the stack.