linux-user: Explicitly untag memory management syscalls

linux-user: Explicitly untag memory management syscalls

We define target_mmap et al as untagged, so that they can be
used from the binary loaders.  Explicitly call cpu_untagged_addr
for munmap, mprotect, mremap syscall entry points.

Add a few comments for the syscalls that are exempted by the
kernel's tagged-address-abi.rst.

Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 20210212184902.1251044[email protected]
Signed-off-by: Peter Maydell <[email protected]>