linux-user: Support target-to-host translation of mlockall argument

The argument to the mlockall system call is not necessarily the same on
all platforms and thus may require translation prior to passing to the
host.

For example, PowerPC 64 bit platforms define values for MCL_CURRENT
(0x2000) and MCL_FUTURE (0x4000) which are different from Intel platforms
(0x1 and 0x2, respectively)

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>