gdb/doc: fix install-html with Texinfo 7

Starting with Texinfo 7 (this commit [1]), the output directory for the
HTML doc format is gdb/doc/gdb_html, rather than gdb/doc/gdb previously.
This breaks the install-html target, which expects the HTML doc to be in
gdb/doc/gdb:

    $ make install-html MAKEINFO=makeinfo DESTDIR=/tmp/install
    make[1]: Entering directory '/home/simark/build/binutils-gdb/gdb'
    make[2]: Entering directory '/home/simark/build/binutils-gdb/gdb/doc'
    makeinfo  -DHAVE_MAKEINFO_CLICK --html  -I /home/simark/src/binutils-gdb/gdb/doc/../../readline/readline/doc -I /home/simark/src/binutils-gdb/gdb/doc/../mi -I /home/simark/src/binutils-gdb/gdb/doc /home/simark/src/binutils-gdb/gdb/doc/gdb.texinfo
    makeinfo  -DHAVE_MAKEINFO_CLICK --html  -I /home/simark/src/binutils-gdb/gdb/doc /home/simark/src/binutils-gdb/gdb/doc/stabs.texinfo
    makeinfo  -DHAVE_MAKEINFO_CLICK --html  -I /home/simark/src/binutils-gdb/gdb/doc /home/simark/src/binutils-gdb/gdb/doc/annotate.texinfo
    test -z "/usr/local/share/doc/gdb" || /bin/sh /home/simark/src/binutils-gdb/gdb/doc/../../mkinstalldirs "/tmp/install/usr/local/share/doc/gdb"
     /usr/bin/install -c -m 644 '/home/simark/src/binutils-gdb/gdb/doc/gdb' '/tmp/install/usr/local/share/doc/gdb/gdb'
    /usr/bin/install: cannot stat '/home/simark/src/binutils-gdb/gdb/doc/gdb': No such file or directory
     /usr/bin/install -c -m 644 '/home/simark/src/binutils-gdb/gdb/doc/stabs' '/tmp/install/usr/local/share/doc/gdb/stabs'
    /usr/bin/install: cannot stat '/home/simark/src/binutils-gdb/gdb/doc/stabs': No such file or directory
     /usr/bin/install -c -m 644 '/home/simark/src/binutils-gdb/gdb/doc/annotate' '/tmp/install/usr/local/share/doc/gdb/annotate'
    /usr/bin/install: cannot stat '/home/simark/src/binutils-gdb/gdb/doc/annotate': No such file or directory
    make[2]: *** [Makefile:278: install-html] Error 1
    make[2]: Leaving directory '/home/simark/build/binutils-gdb/gdb/doc'
    make[1]: *** [Makefile:2240: subdir_do] Error 1
    make[1]: Leaving directory '/home/simark/build/binutils-gdb/gdb'
    make: *** [Makefile:2006: install-html] Error 2

Fix this by adding -o switches to the HTML targets, to force the output
directories.

[1] https://git.savannah.gnu.org/cgit/texinfo.git/commit/?id=a868421baf9c44227c43490687f8d6b8d6c95414

Change-Id: Ie147dc7b4a52eb2348005b8dc006a41b0784621f