target/arm/arm-semi: Correct comment about gdb syscall races

target/arm/arm-semi: Correct comment about gdb syscall races

In arm_gdb_syscall() we have a comment suggesting a race
because the syscall completion callback might not happen
before the gdb_do_syscallv() call returns. The comment is
correct that the callback may not happen but incorrect about
the effects. Correct it and note the important caveat that
callers must never do any work of any kind after return from
arm_gdb_syscall() that depends on its return value.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190916141544.17540-4-peter.maydell@linaro.org