gdbstub: Only build libgdb_user.fa / libgdb_softmmu.fa if necessary

gdbstub: Only build libgdb_user.fa / libgdb_softmmu.fa if necessary

It is pointless to build libgdb_user.fa in a system-only build
(or libgdb_softmmu.fa in a user-only build). Besides, in some
restricted build configurations, some APIs might be restricted /
not available. Example in a KVM-only builds where TCG is disabled:

  $ ninja qemu-system-x86_64
  [99/2187] Compiling C object gdbstub/libgdb_user.fa.p/user.c.o
  FAILED: gdbstub/libgdb_user.fa.p/user.c.o
  ../../gdbstub/user.c: In function ‘gdb_breakpoint_insert’:
  ../../gdbstub/user.c:438:19: error: implicit declaration of function ‘cpu_breakpoint_insert’; did you mean ‘gdb_breakpoint_insert’? [-Werror=implicit-function-declaration]
    438 |             err = cpu_breakpoint_insert(cpu, addr, BP_GDB, NULL);
        |                   ^~~~~~~~~~~~~~~~~~~~~
        |                   gdb_breakpoint_insert
  ../../gdbstub/user.c:438:19: error: nested extern declaration of ‘cpu_breakpoint_insert’ [-Werror=nested-externs]
  ../../gdbstub/user.c: In function ‘gdb_breakpoint_remove’:
  ../../gdbstub/user.c:459:19: error: implicit declaration of function ‘cpu_breakpoint_remove’; did you mean ‘gdb_breakpoint_remove’? [-Werror=implicit-function-declaration]
    459 |             err = cpu_breakpoint_remove(cpu, addr, BP_GDB);
        |                   ^~~~~~~~~~~~~~~~~~~~~
        |                   gdb_breakpoint_remove
  ../../gdbstub/user.c:459:19: error: nested extern declaration of ‘cpu_breakpoint_remove’ [-Werror=nested-externs]
  cc1: all warnings being treated as errors
  ninja: build stopped: subcommand failed.

Fixes: 61b2e136db ("gdbstub: only compile gdbstub twice for whole build")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230329161852.84992-1-philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230403134920.2132362-3-alex.bennee@linaro.org>