Expand fpusave union to include x86_64 versions

Expand fpusave union to include x86_64 versions

The structure of envmmx remains a constant size, but changes composition
between the i386 and x86_64 versions.  While here, flesh out the
832-byte saveymm structure, also distinguishing between 32 and 64 bit
versions.

While here, convert u_char, u_short, u_int16_t, u_int32_t to standard
integers; <sys/types.h> with it's redundant guards is no longer required
although <stdint> is now needed.

This allows the fpu context to be seen on x86_64 by casting the fpusave
union to the new savexmm64 and saveymm64 structures.