kernel: Guard SYS_CLOCK_HW_CYCLES_PER_SEC to avoid spurious empty macro

If SYS_CLOCK_EXISTS is not enabled, then the SYS_CLOCK_HW_CYCLES_PER_SEC
still gets created, but with no value. This causes the code generation
in misc/generated/CMakeLists.txt to create an empty assembly macro:

`.equ  CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC,`

which then causes a build error.

Disable SYS_CLOCK_HW_CYCLES_PER_SEC entirely when SYS_CLOCK_EXISTS is
disabled to fix this.

This is a follow-up to 03f46db8.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>