stabs.texinfo update for macro define/undefine

[David Taylor <dtaylor at emc dot com>]

This relates to my stabs and define/undef patches posted to
gcc-patches and gdb-patches.

The following still needs to be texified.  But, before worrying about
that, I'd like to get comments on the contents.  In the patch and in
the .s file extract below, N_MAC_DEFINE and N_MAC_UNDEF have the
values 0x36 and 0x3a, respectively.  I'm also unsure *where* in
stabs.texinfo to put this information.

Macro define and undefine.

Sometimes macro define and undefine information is recorded in stabs
(e.g., with -g3 -gstabs when using gcc).

A ``#define macro-name macro-body'' is represented with an
N_MAC_DEFINE stab with a `string' field of ``macro-name macro-body''.

An ``#undef macro-name'' is represented with an N_MAC_UNDEF stabs with
a `string' field of simply ``macro-name''.

For both N_MAC_DEFINE and N_MAC_UNDEF, the `desc' field is the line
number within the file where the corresponding #define or #undef
occurred.

For example, the following C code:

    #define NONE	42
    #define TWO(a, b)	(a + (a) + 2 * b)
    #define ONE(c)	(c + 19)

    main(int argc, char *argv[])
    {
      func(NONE, TWO(10, 11));
      func(NONE, ONE(23));

    #undef ONE
    #define ONE(c)	(c + 23)

      func(NONE, ONE(-23));

      return (0);
    }

    int global;

    func(int arg1, int arg2)
    {
      global = arg1 + arg2;
    }

produces the following stabs (as well as many others):

    .stabs	"NONE 42",54,0,1,0                      # 54 is N_MAC_DEFINE
    .stabs	"TWO(a,b) (a + (a) + 2 * b)",54,0,2,0   # 54 is N_MAC_DEFINE
    .stabs	"ONE(c) (c + 19)",54,0,3,0              # 54 is N_MAC_DEFINE
    .stabs	"ONE",58,0,10,0                         # 58 is N_MAC_UNDEF
    .stabs	"ONE(c) (c + 23)",54,0,11,0             # 54 is N_MAC_DEFINE