x86/entry/64: Get rid of the ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR_REGS macros

Previously, error_entry() and paranoid_entry() saved the GP registers
onto stack space previously allocated by its callers. Combine these two
steps in the callers, and use the generic PUSH_AND_CLEAR_REGS macro
for that.

This adds a significant amount ot text size. However, Ingo Molnar points
out that:

	"these numbers also _very_ significantly over-represent the
	extra footprint. The assumptions that resulted in
	us compressing the IRQ entry code have changed very
	significantly with the new x86 IRQ allocation code we
	introduced in the last year:

	- IRQ vectors are usually populated in tightly clustered
	  groups.

	  With our new vector allocator code the typical per CPU
	  allocation percentage on x86 systems is ~3 device vectors
	  and ~10 fixed vectors out of ~220 vectors - i.e. a very
	  low ~6% utilization (!). [...]

	  The days where we allocated a lot of vectors on every
	  CPU and the compression of the IRQ entry code text
	  mattered are over.

	- Another issue is that only a small minority of vectors
	  is frequent enough to actually matter to cache utilization
	  in practice: 3-4 key IPIs and 1-2 device IRQs at most - and
	  those vectors tend to be tightly clustered as well into about
	  two groups, and are probably already on 2-3 cache lines in
	  practice.

	  For the common case of 'cache cold' IRQs it's the depth of
	  the call chain and the fragmentation of the resulting I$
	  that should be the main performance limit - not the overall
	  size of it.

	- The CPU side cost of IRQ delivery is still very expensive
	  even in the best, most cached case, as in 'over a thousand
	  cycles'. So much stuff is done that maybe contemporary x86
	  IRQ entry microcode already prefetches the IDT entry and its
	  expected call target address."[*]

[*] http://lkml.kernel.org/r/20180208094710.qnjixhm6hybebdv7@gmail.com



The "testb $3, CS(%rsp)" instruction in the idtentry macro does not need
modification. Previously, %rsp was manually decreased by 15*8; with
this patch, %rsp is decreased by 15 pushq instructions.

[jpoimboe@redhat.com: unwind hint improvements]

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dan.j.williams@intel.com
Link: http://lkml.kernel.org/r/20180211104949.12992-7-linux@dominikbrodowski.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>