Improved GDB support

[linxuanm]

This PR cleans up the .gdb_init script and adds the vsreg command to print the value stack when given %vsp and %vfp (used in place of vsframe in SPC mode as the value stack bounds are not synced to the interpreter frame).

(gdb) vsreg $r11 $rsi

VFP = 0x7ffdf7f63000, VSP = 0x7ffdf7f630a0
Printing 5 stack values

@ 0x7ffdf7f63080:
  Tag:   0x64
  Value: 0x00000000 0x00000000 0x00000000 0x00000000
@ 0x7ffdf7f63060:
  Tag:   0x64
  Value: 0x00000000 0x00000000 0x00000000 0x00000000
@ 0x7ffdf7f63040:
  Tag:   0x64
  Value: 0x0841f0a0 0x00000000 0x00000000 0x00000000
@ 0x7ffdf7f63020:
  Tag:   0x7f
  Value: 0x0000002c 0x00000000 0x00000000 0x00000000
@ 0x7ffdf7f63000:
  Tag:   0x7e
  Value: 0x00000016 0x00000000 0x00000000 0x00000000

[titzer]

Nice. Can we make a little utility that when run uses the layout description to generate this part of the file? Then it could be part of the make system and automatically update this file.

[linxuanm]

Should this be done with a Virgil script that includes the rest of Wizard as a dependency in order to get the offsets and tag configurations?

[titzer]

Yes. A file like gdbgen.main.v3 that can be run to output the constants should suffice. Or it could be an option to the main wizeng binary to dump out the constants.