[gdb/doc] Escape the '@' symbols in generated texinfo files.

[gdb/doc] Escape the '@' symbols in generated texinfo files.

'@' is a special symbol meaning 'command' in GNU texinfo.

If the GDBINIT or GDBINIT_DIR path during configuration
included an '@' character, the makeinfo command would fail,
as it interpreted the '@' in the path as a start of a command
when expanding the path in the docs.

This patch simply escapes any '@' characters in the path,
by replacing them with '@@'. This was already done for the
bugurl variable.

This was detected because the 'Jenkins' tool sometimes puts
an '@' in the workspace path.

Approved-By: Tom Tromey <tom@tromey.com>