semihosting: fix inline assembly output dependency

Commit d8f186aa ("arch: common: semihost: add semihosting
operations") encapsulated semihosting invocation in a per-arch
semihost_exec() function. There is a fixed register variable declaration
for the return value but this variable is not listed as an output
operand to respective inline assembly segments which is an error.
This is not reported as such by gcc and the generated code is still OK
in those particular instances but this is not guaranteed, and clang
does complain about such cases.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>