Add unit tests for semihosting features

[PhilippvK]

While minimal semihosting capabilities (SYS_WRITE to stdout) can be tested easily with a printf, there are many semihosting features which are tested rarely but still important for virtual prototyping:

• SYS_WRITE/SYS_READ/SYS_READC/SYS_WRITEC
  • Special file descriptors: stdin/stdout/stderr
  • Disk file descriptors
  • Custom FIFOs (Named Pipes)
• SYS_EXIT/SYS_EXIT_EXTENDED (Dealing with EXIT code of target #136)
• SYS_OPEN/SYS_CLOSE
• SYS_SEEK/SYS_FLEN/SYS_ISTTY
• SYS_TIME/SYS_CLOCK/SYS_ELAPSED/SYS_TICKFREQ
• SYS_RENAME/SYS_REMOVE
• SYS_TMPNAM
• SYS_GET_CMDLINE (Semihosting: Pass argv+argv to target (via SYS_GET_CMDLINE) #184)
• ...

I hope I can come up with a test of programs + host-side bash scripts to check if the features are working as expected.