Define register_t using bits/typesizes.h macros.

Currently register_t is, unlike all other types in sys/types.h,
defined using a GCC extension (__attribute__((mode(word)))), falling
back to ‘int’ if the extension is unavailable.  This is a potential
ABI compatibility hazard for people using non-GNU compilers with
glibc.  It’s also unnecessary; the bits/typesizes.h mechanism can
handle all of the existing variation in the definition.  In most
cases, defining __REGISTER_T_TYPE as __SWORD_TYPE is sufficient.
Special handling is necessary for MIPS n32 and x86-64 x32, where
__SWORD_TYPE is ‘int’ and the appropriate type for register_t is
‘long long’.  Unfortunately, this means we need to create a new
bits/typesizes.h variant for linux/mips.  This variant is based
on the top-level bits/typesizes.h, not linux/generic/bits/typesizes.h,
to match the existing MIPS ABIs.

Tested using build-many-glibcs.  The c++-types test confirms that the
physical type of register_t does not change on any supported platform.

	* posix/sys/types.h: Typedef register_t as __register_t.
        * posix/bits/types.h: Typedef __register_t using __REGISTER_T_TYPE.

	* bits/typesizes.h
        * sysdeps/mach/hurd/bits/typesizes.h
        * sysdeps/unix/sysv/linux/alpha/bits/typesizes.h
        * sysdeps/unix/sysv/linux/generic/bits/typesizes.h
        * sysdeps/unix/sysv/linux/s390/bits/typesizes.h
        * sysdeps/unix/sysv/linux/sparc/bits/typesizes.h:
        Define __REGISTER_T_TYPE as __SWORD_TYPE.

        * sysdeps/unix/sysv/linux/mips/bits/typesizes.h:
        New file (copied from bits/typesizes.h).
        Define __REGISTER_T_TYPE as __SWORD_TYPE for o32 and n64 ABIs.
        Define __REGISTER_T_TYPE as __SQUAD_TYPE for n32.
        * sysdeps/unix/sysv/linux/x86/bits/typesizes.h:
        Define __REGISTER_T_TYPE as __SWORD_TYPE for o32 and 64-bit ABIs.
        Define __REGISTER_T_TYPE as __SQUAD_TYPE for x32.

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