parse.c (write_dollar_variable): call new function target_map_name_to_register

parse.c (write_dollar_variable): call new function target_map_name_to_register
	so that targets can define their own register name aliases.
infcmd.c (registers_info): call target_map_name_to_register so that
	"print $reg" and "info reg $reg" share the same register alias set.
mips-tdep.c: separate MIPS_R5900_REGS from NUM_REGS so that sky registers
	can be printed separately.
txvu-tdep.c: print registers according to current CPU context.
tm-txvu.h: define SKY registers and conditionalize register interpretation
	macros.
txvu.mt: Don't bother building remote-mips.o for sky target.