Support -g debug info generation

Support -g debug info generation

-Add GenLineNoSym(), which will generate debug
 symbols for file names and line numbers when
 debug info generation is requested.

-Replace code that warns -g is not supported with
 code to set a flag.

-Complain if -g is specified for non-BSD output
 formats, as only stabs-in-symbol-table/a.out
 format debug information is supported currently.

-Document -g flag in usage information function.

-Document -g flag in manual.

v2:
-Only call debug symbol generation functions when
 dbg_sym != 0