duplicate generic bits/shm.h for each arch using it, in prep to change

duplicate generic bits/shm.h for each arch using it, in prep to change

there are more archs sharing the generic 64-bit version of the struct,
which is uniform and much more reasonable, than sharing the current
"generic" one, and depending on how time64 sysvipc is done for 32-bit
archs, even more may be sharing the "64-bit version" in the future.

so, duplicate the current generic to all archs using it (arm, i386,
m68k, microblaze, or1k) so that the generic can be changed freely.

this is recorded as its own commit mainly as a hint to git tooling, to
assist in copy/move tracking.