arm: smh: Make semihosting trap calls more robust

Commit f4b540e2("arm: smh: Fix uninitialized parameters with newer
GCCs") added a memory clobber to the semihosting inline assembly trap
calls, to avoid too eager GCC optimisation: when passing a pointer, newer
compilers couldn't be bothered to actually fill in the structure that it
pointed to, as this data would seemingly never be used (at least from the
compiler's point of view).
But instead of the memory clobber we need to tell the compiler that we are
passing an *array* instead of some generic pointer, this forces the
compiler to actually populate the data structure.
This involves some rather hideous cast, which is best hidden in a macro.

But regardless of that, we actually need the memory clobber, but for two
different reasons: explain them in comments.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>