mmap() is allowed to have VM_EXEC

The kernel driver was making sure that mmap( "/dev/sysdig0" ) was getting
userspace memory with the (vma->vm_flags & VM_EXEC) flag clear. For whatever
reason this is not true on ARM: VM_EXEC is set there. I can't tell why yet, but
things work fine after removing this check, and it seems benign enough.
Driver makes sure syscall tracepoints are enabled

The sysdig kernel driver is mostly useless without syscall tracepoints. On ARM
these were added relatively recently: in Linux 3.7. This patch explicitly checks
this support exists, and refuses to build if it isn't