Various header changes

Clean changes from my `diffHeaders` branch (which was not based on the correct master, and thus contained lots of irrelevant changes, when one attempted to submit a pull request).

Adds limits.h and stdint.h.
Adds constants to math.h.
Adds ptrdiff_t to stddef.h.
GCC provides this header. `$PREFIX/lib/gcc/$TARGET/$VERSION/include/stdint.h` There's no reason for us to duplicate it.