BACKPORT: linux/clocksource.h: Extract common header for vDSO

The vDSO library should only include the necessary headers required for
a userspace library (UAPI and a minimal set of kernel headers). To make
this possible it is necessary to isolate from the kernel headers the
common parts that are strictly necessary to build the library.

Split clocksource.h into linux and common headers to make the latter
suitable for inclusion in the vDSO library.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200320145351.32292-9-vincenzo.frascino@arm.com


(cherry picked from commit 14ee2ac618e49e767c9af4aa007f951eb9e35153)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I685711da05a1fe1414b93064e9be8b7f9888d8c0