target/mips: convert UHI_plog to use common semihosting code

target/mips: convert UHI_plog to use common semihosting code

Rather than printing directly to stdout lets use our common
semihosting code. There is one minor difference in that the output
currently defaults to stderr instead of stdout however this can be
controlled by connecting semihosting to a chardev.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>