[x86] Make arch_delay_loop() static inline.

Currently for x86, we dont have low-level implementation of
arch_delay_loop(). Instead we have it as non-inlined function in
arch_delay.h. This prevents us from including arch_delay.h at
multiple locations for x86 hence for time-being make this
function as static inline for x86.

Signed-off-by: Anup Patel <anup@brainfault.org>