kgdb: Remove "weak" from kgdb_arch_pc() declaration

kernel/debug/debug_core.c provides a default kgdb_arch_pc() definition
explicitly marked "weak".  Several architectures provide their own
definitions intended to override the default, but the "weak" attribute on
the declaration applied to the arch definitions as well, so the linker
chose one based on link order (see 10629d71 ("PCI: Remove __weak
annotation from pcibios_get_phb_of_node decl")).

Remove the "weak" attribute from the declaration so we always prefer a
non-weak definition over the weak one, independent of link order.

Fixes: 688b744d ("kgdb: fix signedness mixmatches, add statics, add declaration to header")
Tested-by: Vineet Gupta <vgupta@synopsys.com>	# for ARC build
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Harvey Harrison <harvey.harrison@gmail.com>