meson: Introduce target-specific Kconfig

Add a target-specific Kconfig. We need the definitions in Kconfig so
the minikconf tool can verify they exits. However CONFIG_FOO is only
enabled for target foo via the meson.build rules.

Two architecture have a particularity, ARM and MIPS. As their
translators have been split you can potentially build a plain 32 bit
build along with a 64-bit version including the 32-bit subset.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210131111316.232778-6-f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210707131744.26027-2-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>