timer: Decouple x86_64 specific timer_gettime from generic Linux implementation

The x86_64 specific timer_gettime implementation (from
./linux/x86_64/timer_gettime.c) reused the Linux generic one (from
./linux/timer_gettime.c) to implement handling some compatible timers
(previously defined in librt, now in libc).

As the generic implementation now is going to also support new (available
from Linux 5.1+) timer_gettime64 syscall, those two implementations have
been decoupled for easier conversion.

The original INLINE_SYSCALL() macro has been replaced with
INLINE_SYSCALL_CALL() to avoid explicit passing the number of arguments.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>