Fix memory tagging section type

Fix memory tagging section type

It was reported to me that on Ubuntu 14.04 (fairly old) the documentation
fails to build with the following:

gdb/doc/gdb.texinfo:10888: warning: node `Memory' is up for `Memory Tagging' in sectioning but not in menu
gdb/doc/gdb.texinfo:10693: node `Memory' lacks menu item for `Memory Tagging' despite being its Up target
Makefile:491: recipe for target 'gdb.info' failed
make[3]: *** [gdb.info] Error 1

This doesn't seem to happen on Ubuntu 18.04/20.04, but it does make sense.

Fix this by turning @subsection into a @section and adding the
"Memory Tagging" entry to the menu.

gdb/doc/ChangeLog:

2021-03-29  Luis Machado  <luis.machado@linaro.org>

	* gdb.textinfo (Memory Tagging): Make it a @section.