Try to locate syscall instruction in vdso mapping where possible

[bjorn3]

This is faster than overwriting a piece of executable memory with a syscall instruction while preserving the original value, diverting execution to it, running the syscall, restoring all registers and then writing the original code back. Instead it is possible to search for a syscall instruction in the vdso mapping once and then every time a syscall needs to be made to divert execution to this instruction, running the syscall and then restoring all registers as it saves 3 ptrace syscalls.