Fix stalker memory overrun for trust_threshold of -1

With this threshold we will never re-cycle any blocks, so instead of
relying on an allocator thread adding more slabs we will simply just
wrap around and overwrite the first block we generated.
Skip the ret instruction as we replace it with our own code

Not really an issue since a block always ends at a ret, but improves
consistency.
Restore edx register after sysenter completes
Fix warning reported by clang
Fix gum_mnemonic_is_jcc for the jno instruction