[NFC] [libomptarget] Move option.h into target_impl.h, name a magic number
Essentially everything under option.h is target dependent. This change moves the
existing code and names a mask that depends on the warp/wavefront size.
The macros could be changed to enums / functions / deleted subsequently.
Note that this change from 0x1F to a function of warp size means the mask is 0x3F on 64 wide systems.