auxilary/os: correct sysctl use in os_get_total_physical_memory()

auxilary/os: correct sysctl use in os_get_total_physical_memory()

The length argument passed to sysctl was the size of the pointer
not the type.  The result of this is sysctl calls would fail on
32 bit BSD/Mac OS X.

Additionally the wrong pointer was passed as an argument to store
the result of the sysctl call.

Cc: "10.4, 10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>