x86-64: make BUILD_IRQ() also reset section back

Commit 9d25d4db

 ("x86: BUILD_IRQ say
.text to avoid .data.percpu") added a ".text" specifier to make sure
that BUILD_IRQ() builds the irq trampoline in the text segment rather
than in some random left-over segment that the compiler happened to
leave the asm in.

However, we should also make sure that we switch back by adding a
".previous" at the end, so that there are no subtle issues with
subsequent compiler-generated code.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>