Similarly to the 'target_softmmu_arch' source set which allows
to restrict target-specific sources to system emulation, add
the equivalent 'target_user_arch' set for user emulation.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-12-f4bug@amsat.org>