x86/extable: Macrofy inline assembly code to work around GCC inlining bugs

As described in:

  77b0bf55bc67: ("kbuild/Makefile: Prepare for using macros in inline assembly code to work around asm() related GCC inlining bugs")

GCC's inlining heuristics are broken with common asm() patterns used in
kernel code, resulting in the effective disabling of inlining.

The workaround is to set an assembly macro and call it from the inline
assembly block - which is also a minor cleanup for the exception table
code.

Text size goes up a bit:

      text     data     bss      dec     hex  filename
  18162555 10226288 2957312 31346155 1de4deb  ./vmlinux before
  18162879 10226256 2957312 31346447 1de4f0f  ./vmlinux after (+292)

But this allows the inlining of functions such as nested_vmx_exit_reflected(),
set_segment_reg(), __copy_xstate_to_user() which is a net benefit.

Tested-by: Kees Cook <[email protected]>
Signed-off-by: Nadav Amit <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/lkml/[email protected]/T/#u
Signed-off-by: Ingo Molnar <[email protected]>

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