package/freerdp: do not use SSE2 extensions when not available

Do not let FreeRDP decide whether it can use SE2 opcodes, it may well
fail to do so, because the heuristic is not working for
cross-compilation.

Also, we do have a Kconfig option stating whether we have SSE2 or not,
so reuse that.

Similar to the recent ARM+Neon fix.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>