Implement __attribute__((nodecorate))

Implement __attribute__((nodecorate))

Prevent any decoration that would otherwise affect an exported
PE function. For example, given the following:

__declspec(dllexport) __stdcall
int decorated(int arg) {
    return 0;
}

__declspec(dllexport) __stdcall __attribute__((nodecorate))
int undecorated(int arg) {
    return 0;
}

The following exported functions can now be seen in the DLL:
_decorated@4
undecorated

The attribute is recognised for all targets but only
affects PE codegen. I'm not sure whether this would be
useful for other targets; its intended use was to allow
the creation of a DLL matching an existing set of signatures.