target-mips: add Unified Hosting Interface (UHI) support

Add SDBBP-based Unified Hosting Interface (UHI) which is deactivated by default.
When activated, the SDBBP instruction with cause field equal to 1 calls one of
the UHI operations instead of causing a debug exception. Arguments for UHI
operations are passed via general purpose registers:
In:  $25 operation code, $4-$7 operation arguments
Out: $2-3 operation return values

QEMU command line options for semihosting:
"-semihosting" Activates semihosting.
"-semihosting-arg" Adds string to the argument vector passed to main() in a guest program. Can be used multiple times.
"-semihosting-root" Sets new root for a guest. All paths used by a guest are prepended with this string.

Updates for #17 

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>