target/arm: Select CONFIG_ARM_V7M when TCG is enabled

We cannot allow this config to be disabled at the moment as not all of
the relevant code is protected by it.

Commit 29d9efc ("arm/Kconfig: Do not build TCG-only boards on a
KVM-only build") moved the CONFIGs of several boards to Kconfig, so it
is now possible that nothing selects ARM_V7M (e.g. when doing a
--without-default-devices build).

Return the CONFIG_ARM_V7M entry to a state where it is always selected
whenever TCG is available.

Fixes: 29d9efc ("arm/Kconfig: Do not build TCG-only boards on a KVM-only build")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230508181611.2621-3-farosas@suse.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/Kconfig

@@ -1,6 +1,7 @@
 config ARM
     bool
     select ARM_COMPATIBLE_SEMIHOSTING if TCG
+    select ARM_V7M if TCG
 
 config AARCH64
     bool