riscv: Avoid kgdb.h including gdb_xml.h to solve unused-const-variable warning

The constant arrays in gdb_xml.h are only used in arch/riscv/kernel/kgdb.c,
but other c files may include the gdb_xml.h indirectly via including the
kgdb.h. Hence, It will cause many unused-const-variable warnings. This
patch makes the kgdb.h not to include the gdb_xml.h to solve this problem.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

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>

riscv: Use the XML target descriptions to report 3 system registers

The $status, $badaddr, and $cause registers belong to the thread context,
so KGDB can obtain their contents from pt_regs in each trap. However, the
sequential number of these registers in the gdb register list is far from
the general-purpose registers. If riscv port uses the existing method to
report these three registers, many trivial registers with sequence numbers
in the middle of them will also be packaged to the reply packets. To solve
this problem, the riscv port wants to introduce the GDB target description
mechanism to customize the reported register list. By the list, the KGDB
can ignore the intermediate registers and just reports the general-purpose
registers and these three system registers.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

riscv: Add KGDB support

The skeleton of RISC-V KGDB port.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>