Fix crash in syck_emit on platforms with long long pointers

This fixes RT#83879. Strawberry Perl uses the 32-bit version of MinGW, so `MINGW64` is not defined. This replaces the pointer typedefs with `uintptr_t` from `stdint.h`. Another crash came from the fact that `syck_emit` had `st_lookup` write a `long long` to the `long x` which caused part of the `parent` pointer to be overwritten when a reference was being emitted. The `x` variable wasn't actually being used for anything, so I removed it. All tests are now passing on Windows 64-bit.