mips-linux-user: Enable mips64 and mipsn32 linux-user targets

At this point we can enable compilation, though things
still don't work.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -986,6 +986,10 @@ microblaze-linux-user \
 microblazeel-linux-user \
 mips-linux-user \
 mipsel-linux-user \
+mips64-linux-user \
+mips64el-linux-user \
+mipsn32-linux-user \
+mipsn32el-linux-user \
 or32-linux-user \
 ppc-linux-user \
 ppc64-linux-user \