docs: Roll semihosting option information into qemu-options.hx

Currently the per-target documentation for those targets that
implement semihosting includes a bit of text that goes into both the
manual and the manpage about options specific to the target.  This
text is redundant with the earlier generic option description of the
semihosting option produced from qemu-options.hx. To avoid having
to create a lot of stub include files to include into the rST
generated qemu.1 manpage, roll target-specific bits of information
into the qemu-options.hx text, so the user doesn't have to look
in two places for this information.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-24-peter.maydell@linaro.org