ARM: clean up the memset64() C wrapper

The current logic to split the 64-bit argument into its 32-bit halves is
byte-order specific and a bit clunky.  Use a union instead which is
easier to read and works in all cases.

GCC still generates the same machine code.

While at it, rename the arguments of the __memset64() prototype to
actually reflect their semantics.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>