fudge r11 over syscalls on x86-64

Without this patch, step_thread fails with:

(gdb) s
s
[ERROR /home/froydnj/src/rr/src/Registers.cc:289:maybe_print_reg_mismatch() errno: 0 'Success']
 -> r11 0x306 != 0x206 (replaying vs. recorded)
Remote connection closed

The comments around the register fudging explain the rationale.
compile librrpreload as -fPIC always