arch: common: semihost: add semihosting operations

Add an API that utilizes the ARM semihosting mechanism to interact with
the host system when a device is being emulated or run under a debugger.

RISCV is implemented in terms of the ARM implementation, and therefore
the ARM definitions cross enough architectures to be defined 'common'.

Functionality is exposed as a separate API instead of syscall
implementations (`_lseek`, `_open`, etc) due to various quirks with
the ARM mechanisms that means function arguments are not standard.

For more information see:
https://developer.arm.com/documentation/dui0471/m/what-is-semihosting-



Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

impl