x86/fpu: Add CONFIG_X86_DEBUG_FPU=y FPU debugging code

There are various internal FPU state debugging checks that never
trigger in practice, but which are useful for FPU code development.

Separate these out into CONFIG_X86_DEBUG_FPU=y, and also add a
couple of new ones.

The size difference is about 0.5K of code on defconfig:

   text        data     bss          filename
   15028906    2578816  1638400      vmlinux
   15029430    2578816  1638400      vmlinux

( Keep this enabled by default until the new FPU code is debugged. )

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>

arch/x86/Kconfig.debug

@@ -332,4 +332,16 @@ config X86_DEBUG_STATIC_CPU_HAS
 
 	  If unsure, say N.
 
+config X86_DEBUG_FPU
+	bool "Debug the x86 FPU code"
+	depends on DEBUG_KERNEL
+	default y
+	---help---
+	  If this option is enabled then there will be extra sanity
+	  checks and (boot time) debug printouts added to the kernel.
+	  This debugging adds some small amount of runtime overhead
+	  to the kernel.
+
+	  If unsure, say N.
+
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