GCC RISC-V support

[CAS-ual-TY]

This PR changes selfie in such a way that it also compiles for riscv64 gcc and runs on Unicorn.

To compile on a riscv64 cross compiler: riscv64-unknown-elf-gcc selfie.c -o selfie-riscv.o -no-pie -include "stdlib.h" -include "stdio.h" -include "unistd.h" -include "fcntl.h" The resulting selfie-riscv.o runs on Unicorn.

Changes:

Changes to compiler flags:

• include stdint.h
• include unistd.h
• include fcntl.h
• remove D'uint64_t=unsigned long' option (covered by stdint.h)
• add -Wno-sign-compare -Wno-unused-result flags (should not interfere with student assignments)

Changed constants (due to conflict with header files):

• UINT64_MAX to S_UINT64_MAX (conflict in stdint.h)
• INT64_MIN to S_INT64_MIN (conflict in stdint.h)
• INT64_MAX to S_INT64_MAX (conflict in stdint.h)
• O_RDONLY to F_O_RDONLY

Changed method names (due to conflict with header files):

• atoi to ascii_to_int
• itoa to int_to_ascii