kgdb: Move the extern declaration kgdb_has_hit_break() to generic kgdb.h

Currently, only riscv kgdb.c uses the kgdb_has_hit_break() to identify
the kgdb breakpoint. It causes other architectures will encounter the "no
previous prototype" warnings if the compile option has W=1. Moving the
declaration of extern kgdb_has_hit_break() from risc-v kgdb.h to generic
kgdb.h to avoid generating these warnings.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>