fpu/internal.h has grown organically, with not much high level structure,
which hurts its readability.

Organize the various definitions into 5 sections:

 - high level FPU state functions
 - FPU/CPU feature flag helpers
 - fpstate handling functions
 - FPU context switching helpers
 - misc helper functions

Other related changes:

 - Move MXCSR_DEFAULT to fpu/types.h.
 - drop the unused X87_FSW_ES define

No change in functionality.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>