x86/timer: Don't inline __const_udelay()

LTO will happily inline __const_udelay() everywhere it is used.  Forcing it
noinline saves ~44k text in a LTO build.

13999560        1740864 1499136 17239560        1070e08 vmlinux-with-udelay-inline
13954764        1736768 1499136 17190668        1064f0c vmlinux-wo-udelay-inline

Even without LTO this function should never be inlined.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190330004743.29541-4-andi@firstfloor.org