Support HTIF-based semihosting

[PhilippvK]

The standardized RISC-V/ARM Semihosting implementation is nice and mostly works great, but there are some flaws:

• Exit code of a program can not be communicated on 32-bit targets (Dealing with EXIT code of target #136 )
• Many RISC-V baremetal SW is using tohost/fromhost (HTIF, host-target interface) for dealing with syscalls
• Older versions of GCC might not have the semihosting support integrated in libgloss

I propose to implement HTIF-based semihosting as an alternative to improve software compatibility. The code changes shouldn’t be too difficult anyways.

See https://github.com/ucb-bar/libgloss-htif for details