Add gcc argument to fix GDB error

Add the `-gdwarf2` argument to gcc to fix an issue when debugging in
GDB. Without this argument,you'd get the following error when trying to
look at variables:

    <can't compute CFA for this frame>
Add initial implementation of `dup2` system call

Add initial implementation of `dup2` system call. Currently we expect to
have an open file descriptor to clone into
Add userspace test program for dup2 system call
Complete `dup2` system call

Add support for duplicating a file descriptor if the destination file
descriptor does not yet exist. Also adjust the spacing to 4 space tabs
Modify dup2_test program for new syscall functionality

Modify the dup2_test program to also test `dup2` functionality if our
destination file descriptor has not been opened.
Adjust README.md