[BUG] File open does not have the append flag

[martinvuyk]

Bug description

File open does not have the append flag.

Steps to reproduce

This works in python but doesn't in Mojo

from os import remove

def main():
    name = "some_file.txt"

    with open(name, "w") as f:
        f.write("new contents 🔥")

    with open(name, "r") as f:
        contents = f.read()
        print(contents)

    with open(name, "a") as f:
        f.write("add another fire 🔥")

    with open(name, "r") as f:
        contents = f.read()
        print(contents)
    
    remove(name)

This is a compiler issue :

var handle = external_call[
    "KGEN_CompilerRT_IO_FileOpen", OpaquePointer
](path, mode, Pointer.address_of(err_msg))

System information

- Provide version information for Mojo by pasting the output of `mojo -v`
`mojo 24.6.0.dev2024120805`