Add NewDebugSymbol(): stabs symbol factory

This function, currently unused, generates a stabs
debugging symbol, as documented here:

https://sourceware.org/gdb/onlinedocs/stabs.html

It can be used to process stabs directives, also
documented at the above URL, generated by HLL
compilers such as GCC, as well as to generate line
number and file name debug symbols when assembling
hand-coded assembly files with the -g option.

v2:
-Don't double-init stabs symbol fields
-Consistently use tabs, not spaces