* eval.c (struct backtrace): Simplify and port the data structure.

* eval.c (struct backtrace): Simplify and port the data structure.

Do not assume that "int nargs : BITS_PER_INT - 2;" produces a
signed bit field, as this assumption is not portable and it makes
Emacs crash when compiled with Sun C 5.8 on sparc.  Do not use
"char debug_on_exit : 1" as this is not portable either; instead,
use the portable "unsigned int debug_on_exit : 1".  Remove unused
member evalargs.  Remove obsolete comments about cc bombing out.