Use a constructor function to create regset structures.

Use a constructor function to create regset structures.
* regset.h (supply_regset_ftype, collect_regset_ftype): New typedefs.
(struct regset): Use supply_regset_ftype.  Add new
'collect_regset' member.
(regset_xmalloc): New declaration.
* regset.c: New file.
* am64-tdep.c (amd64_regset_from_core_section): Use
regset_xmalloc to construct regset structures.
* amd64obsd-tdep.c (amd64obsd_regset_from_core_section): Same.
* i386-tdep.c (i386_regset_from_core_section): Same.
* i386nbsd-tdep.c (i386nbsd_aout_regset_from_core_section): Same.
* i386obsd-tdep.c (i386obsd_aout_regset_from_core_section): Same.
* sparc64fbsd-tdep.c (sparc64fbsd_init_abi): Same.
* sparc64nbsd-tdep.c (sparc64nbsd_init_abi): Same.
* sparc64obsd-tdep.c (sparc64obsd_init_abi): Same.
* sparcnbsd-tdep.c (sparc32nbsd_init_abi): Same.
* Makefile.in (COMMON_OBS): Add regset.o.
(regset.o): New rule.