gdb: change gdbarch_has_shared_address_space to return bool

Change the target_is_uclinux global in m68k-linux-tdep.c to be an
std::optional, for clarity.

Change-Id: Ifb401865bac15dbb9803aeeb9f3d61ed965d6541
Approved-By: Tom Tromey <tom@tromey.com>
9 files changed