Move userland bits of i386 npx.h and amd64 fpu.h to x86 fpu.h.

Move userland bits of i386 npx.h and amd64 fpu.h to x86 fpu.h.
Remove FPU types from compat/ia32/ia32_reg.h that are no longer needed.
Create machine/npx.h on amd64 to allow compiling i386 code that uses
this header.

The original npx.h and fpu.h define struct envxmm differently. Both
definitions have been included in the new x86 header as struct __envxmm32
and struct __envxmm64. During compilation either __envxmm32 or __envxmm64
is defined as envxmm depending on machine architecture. On amd64 the i386
struct is also available as struct envxmm32.

Reviewed by:	kib