target/arm: Explain why we need to select ARM_V7M

We currently need to select ARM_V7M unconditionally when TCG is
present in the build because some translate.c helpers and the whole of
m_helpers.c are not yet under CONFIG_ARM_V7M.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230523180525.29994-2-farosas@suse.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/Kconfig

@@ -1,6 +1,9 @@
 config ARM
     bool
     select ARM_COMPATIBLE_SEMIHOSTING if TCG
+
+    # We need to select this until we move m_helper.c and the
+    # translate.c v7m helpers under ARM_V7M.
     select ARM_V7M if TCG
 
 config AARCH64