linux: Simplify clock_getres

With arch-syscall.h it can now assumes the existance of either
__NR_clock_getres or __NR_clock_getres_time64.  The 32-bit time_t
support is now only build for !__ASSUME_TIME64_SYSCALLS.

It also uses the time64-support functions to simplify it further.

Checked on x86_64-linux-gnu and i686-linux-gnu (on 5.4 and on 4.15
kernel).

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>