Handle -fpic thunks

[Kenzzer]

Close #90

On linux, the x86 binaries compiled with gcc's -fPIC option will often have a function prologue like this

push    ebp
mov     ebp, esp
push    edi
call    __x86_get_pc_thunk_di

This is unfortunate, because call __x86_get_pc_thunk_di gets moved into the trampoline. For any other kind of function this would be fine, but here it isn't because here's what __x86_get_pc_thunk does :

mov reg, [esp]
ret

The reg is the one that corresponds to the function name. EAX for AX, ECX for CX, etc...
Now the return address in esp is that of the trampoline not the original function, so when jumping back to the original function the register contains an invalid value..

This is what this PR fixes. If a call to __x86_get_pc_thunk is identified as being moved into the trampoline, instead of copying the original bytes, we copy the opcodes for mov reg, 0xSomeValue where reg is the register that would have been assigned by __x86_get_pc_thunk and 0xSomeValue is the value the register would have had in the original non-detour'd function.