um: merge HOST_... of registers common on i386 and amd64

arch/x86/um/ptrace_32.c

arch/x86/um/ptrace_64.c

arch/x86/um/shared/sysdep/ptrace_32.h

arch/x86/um/shared/sysdep/ptrace_64.h

arch/x86/um/signal_32.c

arch/x86/um/signal_64.c

arch/x86/um/user-offsets.c