Allocate regset structures in the gdbarch's obstack, not using

Allocate regset structures in the gdbarch's obstack, not using
xmalloc.
* regset.c (regset_alloc): Renamed from regset_xmalloc.
Add 'arch' argument.  Allocate the regset on arch's obstack, not
using xmalloc.
* regset.h (regset_alloc): Update declaration.
* am64-tdep.c (amd64_regset_from_core_section): Update call; pass
gdbarch argument.
* 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.