As part of some work in glibc to move away from the "__need" prefix,
this commit breaks away the definitions of __int_reg_t, __uint_reg_t,
__INT_REG_BITS, and __INT_REG_FMT to a separate <arch/intreg.h>
"microheader".  It is then included from <arch/abi.h> to preserve
the semantics of the previous header.

For now, we continue to preserve the __need_int_reg_t semantics
in <arch/abi.h> as well, but anticipate that after a few years
we can obsolete it.