target/arm/gdbstub: Only advertise M-profile features if TCG available

target/arm/gdbstub: Only advertise M-profile features if TCG available

Cortex-M profile is only emulable from TCG accelerator. Restrict
the GDBstub features to its availability in order to avoid a link
error when TCG is not enabled:

  Undefined symbols for architecture arm64:
    "_arm_v7m_get_sp_ptr", referenced from:
        _m_sysreg_get in target_arm_gdbstub.c.o
    "_arm_v7m_mrs_control", referenced from:
        _arm_gdb_get_m_systemreg in target_arm_gdbstub.c.o
  ld: symbol(s) not found for architecture arm64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Fixes: 7d8b28b8b5 ("target/arm: Implement gdbstub m-profile systemreg and secext")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20230322142902.69511-3-philmd@linaro.org
[PMM: add #include since I cherry-picked this patch from the series]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>