texi2pod.pl: import support for @t{...} from gcc

GDB's man page source (in gdb.texinfo) contains:

    @t{++}

The @t{...} part is supposed to display the wrapped text with a
fixed-width font.  The texi2pod.pl script currently doesn't handle
@t{...}, so it appears as-is in the man page:

    You can use GDB to debug programs written in C, C@t{++}, Fortran and Modula-2.

gcc's version of texi2pod.pl (at contrib/texi2pod.pl in gcc's repo)
replaces @t{...} with the wrapped text as-is, which I think is an
acceptable behavior.  The fixed-width font distinction is not really
important for a man page, where the text will be displayed with whatever
font the user is using.

Import the line that does that from gcc's version.

I have verified that there is no other, unwanted change in man pages
generated in binutils' and GDB's doc, with this patch applied.

etc/ChangeLog:

	* texi2pod.pl: Handle @t{...} tags.