x86: Rename "ignore" macro in <asm/dwarf2.h> to avoid collision

Commit 70f1bba4

 ("x86: use ignore macro instead of hash comment") breaks
the 64-bit x86 build on toolchains that have CONFIG_AS_CFI undefined with:

    arch/x86/lib/csum-copy_64.S:48: Error: Macro `ignore' was already defined

because <asm/dwarf2.h> now uses the ignore macro name itself.  Fix this
by changing to __cfi_ignore in dwarf2.h.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>