* monitor.h: Add the function regname to monitor_ops

* monitor.h: Add the function regname to monitor_ops
structure. This way NUM_REGS does not have to be a constant.
* monitor.c (monitor_fetch_register): Added support for regname
function. The function is called if the array regnames is NULL.
(monitor_store_register): Same.
* cpu32bug-rom.c (cpu32bug_regname):  Add function. Replaces regnames array.
(init_cpu32bug_cmds): set cpu32bug_cmds.regnames to NULL,
cpu32bug_cmds.regname to point to new function.
* abug-rom.c (abug_regname): Same as above.
(init_abug_cmds): Same.
* dbug-rom.c (dbug_regname): Same as above.
(init_dbug_cmds): Same.
* remote-est.c (est_regname): Same.
(init_est_cmds): Same.
* rom68k-rom.c (rom68k_regname): Same.
(init_rom68k_cmds): Same.