This change modifies the definition of __int64 and thus of many other
types defined on the basis of it to match the OS definitions. This
ensures that we can use these types in interfaces between code in
coreclr and various PALs that are compiled against OS headers.

The key issue was that we were defining __int64 for 64 bit OSes as
long while Unix defines it as long long. The size of those types is the
same on Unix, but they are different and result in different mangling of
C++ names.