Implement crt0 runtime startup code for userland

[pbalduino]

Context

Part of cross-compiler toolchain (#29). Currently each userland program implements its own _start entry point.

Requirements

• Create src/usermode/crt0.S with _start entry point
• Setup stack and registers
• Call main(argc, argv, envp)
• Exit with proper return code

Dependencies

None

Acceptance Criteria

• crt0.S implements standard _start
• Calls main() correctly
• All userland programs can link against crt0.o