x86: BUILD_IRQ say .text to avoid .data.percpu

When I edit the x86_64 Makefile to -fno-unit-at-a-time, bootup panics
on 0xCCs in IRQ0x3e_interrupt(): IRQ0x20_interrupt etc. have got linked
into .data.percpu.  Perhaps there are other ways of triggering that:
specify ".text" in the BUILD_IRQ() macro for safety.

I've been using -fno-unit-at-a-time (to lessen inlining, for easier
debugging) for a long time.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>