ARM: 8629/1: vfp: properly tag assembly function declarations in C code

This is good for consistency even if there is no difference in compiled
code. LTO might rely on this eventually. No need to preserve the extern
attribute as it is the default with function prototypes.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>