seminaries/qtrvsim/os-emu-example: remap open(at) flags to kernel from library defined ones

This allows to compile and run on Linux kernel even binaries
even binaries linked even with C library build for embedded
targets (bare metal riscv32-unknown-elf or riscv64-unknown-elf
NewLib is usable then).

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>