Add signal number conversions for NetBSD

Add signal number conversions for NetBSD

gdb/ChangeLog:

	* nbsd-tdep.c: Include "gdbarch.h".
	Define enum with NetBSD signal numbers.
	(nbsd_gdb_signal_from_target, nbsd_gdb_signal_to_target): New.
	* alpha-nbsd-tdep.c (alphanbsd_init_abi): Call nbsd_init_abi().
	* amd64-nbsd-tdep.c (amd64nbsd_init_abi): Likewise.
	* arm-nbsd-tdep.c (arm_netbsd_elf_init_abi): Likewise.
	* hppa-nbsd-tdep.c (hppanbsd_init_abi): Likewise.
	* i386-nbsd-tdep.c (i386nbsd_init_abi): Likewise.
	* mips-nbsd-tdep.c (nbsd_init_abi): Likewise.
	* ppc-nbsd-tdep.c (ppcnbsd_init_abi): Likewise.
	* sh-nbsd-tdep.c (shnbsd_init_abi): Likewise.
	* sparc-nbsd-tdep.c (sparc32nbsd_init_abi): Likewise.
	* sparc64-nbsd-tdep.c (sparc64nbsd_init_abi): Likewise.
	* vax-nbsd-tdep.c (vaxnbsd_elf_init_abi): Likewise.