the bits file is retained, but as a single generic version, to allow for the unlikely future possibility of letting a new arch define something differently.
this removes an otherwise-unnecessary bits header from most archs, replacing it with an empty generic version.
future archs should not define their own bits/stat.h but use this generic one.
the fcntl file locking command macro values in the existing generic bits/fcntl.h were the "64" variants, requiring 64-bit archs that use the "plain" variants to have their own bits/fcntl.h, even if they otherwise use the common definitions for everything. since commit 7cc79d10afd43811a486fd5e9fcdf8e45ac599e0 exposed __LONG_MAX to all bits headers, we can now make the generic one common between 32- and 64-bit archs.
this is not necessary for linux but is a simple, inexpensive change to make that facilitates ports to systems where NAME_MAX needs to be longer.
now that all 32-bit archs have 64-bit time types, the values for the time-related ioctls can be shared. the mechanism for this is an arch/generic version of the bits header. archs which don't use the generic header still need to duplicate the definitions. x32, which does not use the new time64 values of the macros, already has its own overrides, so this commit does not affect it.
these structures can now be defined generically in terms of endianness and long size. previously, the 32-bit archs all shared a common definition from the generic bits header, and each 64-bit arch had to repeat the 64-bit version, with endian conditionals if the arch had variants of each endianness. I would prefer getting rid of the preprocessor conditionals for padding and instead using unnamed bitfield members, like commit 9b2921bea1d5017832e1b45d1fd64220047a9802 did for struct timespec. however, at present sendmsg, recvmsg, and recvmmsg need access to the padding members by name to zero them. this could perhaps be cleaned up in the future.
LLONG_MAX is uniform for all archs we support and plenty of header and code level logic assumes it is, so it does not make sense for limits.h bits mechanism to pretend it's variable. LONG_BIT can be defined in terms of LONG_MAX; there's no reason to put it in bits. by moving LONG_MAX definition to __LONG_MAX in alltypes.h and moving LLONG_MAX out of bits, there are now no plain-C limits that are defined in the bits header, so the bits header only needs to be included in the POSIX or extended profiles. this allows the feature test macro logic to be removed from the bits header, facilitating a long-term goal of getting such logic out of bits. having __LONG_MAX in alltypes.h will allow further generalization of headers. archs without a constant PAGESIZE no longer need bits/limits.h at all.
otherwise, 32-bit archs that could otherwise share the generic bits/ipc.h would need to duplicate the struct ipc_perm definition, obscuring the fact that it's the same. sysvipc is not widely used and these headers are not commonly included, so there is no performance gain to be had by limiting the number of indirectly included files here. files with the existing time32 definition of IPC_STAT are added to all current 32-bit archs now, so that when it's changed the change will show up as a change rather than addition of a new file where it's less obvious that the value is changing vs the generic one that was used before.
the definition of the IPC_64 macro controls the interface between libc and the kernel through syscalls; it's not a public API. the meaning is rather obscure. long ago, Linux's sysvipc *id_ds structures used 16-bit uids/gids and wrong types for a few other fields. this was in the libc5 era, before glibc. the IPC_64 flag (64 is a misnomer; it's more like 32) tells the kernel to use the modern[-ish] versions of the structures. the definition of IPC_64 has nothing to do with whether the arch is 32- or 64-bit. rather, due to either historical accident or intentional obnoxiousness, the kernel only accepts and masks off the 0x100 IPC_64 flag conditional on CONFIG_ARCH_WANT_IPC_PARSE_VERSION, i.e. for archs that want to provide, or that accidentally provided, both. for archs which don't define this option, no masking is performed and commands with the 0x100 bit set will fail as invalid. so ultimately, the definition is just a matter of matching an arbitrary switch defined per-arch in the kernel.