add EXTA, EXTB, CIBAUD, CMSPAR, XCASE macros and hide them as well as
CBAUD, ECHOCTL, ECHOPRT, ECHOKE, FLUSHO, PENDIN in standard mode.
the new macros are both in glibc termios.h and in linux asm/termbits.h,
the later also contains IBSHIFT and BOTHER, those were not added.
these are not standard macros, but some of them are in the reserved
namespace so could be exposed, the ones which are not reserved are
CIBAUD, CMSPAR and XCASE (which was removed in issue 6), the rest
got hidden to be consistent with glibc.
all bits headers that were identical for a number of 'clean' archs are
moved to the new arch/generic tree. in addition, a few headers that
differed only cosmetically from the new generic version are removed.
additional deduplication may be possible in mman.h and in several
headers (limits.h, posix.h, stdint.h) that mostly depend on whether
the arch is 32- or 64-bit, but they are left alone for now because
greater gains are likely possible with more invasive changes to header
logic, which is beyond the scope of this commit.
