Replace __attribute_pure__ with __pure

To be consistent with the use of attributes in the rest of the kernel
replace all use of __attribute_pure__ with __pure and delete the definition
of __attribute_pure__.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Acked-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Bryan Wu <bryan.wu@analog.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>