package/gdb: switch to gdb 10.x as the default

When doing this, we can simplify the RISC-V related logic. Indeed, as
there was no support for RISC-V in gdb 9.x, which was the current
default, we had some trickery in the Config.in file to ensure gdb 10.x
was used by default on RISC-V. However now that 10.x is the default
for everybody, this trickery is no longer needed.

Also, we now needs to enable BR2_PACKAGE_GDB_TOPLEVEL when host-gdb is
not built, as the default target gdb version is 10.x, and threfore
requires BR2_PACKAGE_GDB_TOPLEVEL=y.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>