tools/Rules.mk: Don't optimize debug builds; add macro debugging information

tools/Rules.mk: Don't optimize debug builds; add macro debugging information

Tools debug builds are built with optimization level -O1, inherited from
the CFLAGS definition in StdGNU.mk.   Optimizations confuse the debugger,
and the comment justifying -O1 in StdGNU.mk should not apply for a
userspace library.   Disable optimization by appending -O0 to CFLAGS,
which overrides the -O1 flag specified earlier.

Also specify -g3, to add macro debugging information which allows
gdb to expand macro invocations.   This is useful as libxl uses many
non-trivial macros.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- inserted a clarifying "enable" into comment ]