As per the spec:

  the PARAMETER REGISTER contains the address of a pointer to a
  four-field data block.

So we need to follow arg0 and place the results of SYS_HEAPINFO there.

Fixes: 3c37cfe0 ("semihosting: Change internal common-semi interfaces to use CPUState *")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Cc: Bug 1915925 <1915925@bugs.launchpad.net>
Cc: Keith Packard <keithp@keithp.com>
Bug: https://bugs.launchpad.net/bugs/1915925
Message-Id: <20210323165308.15244-8-alex.bennee@linaro.org>