um: Fix passing '-n' to linker for stub_exe

um: Fix passing '-n' to linker for stub_exe

When building stub_exe with clang, there is an error because '-n' is not
a recognized flag by the clang driver (which is being used to invoke the
linker):

  clang: error: unknown argument: '-n'

'-n' should be passed along to the linker, as it is the short flag for
'--nmagic', so prefix it with '-Wl,'.

Fixes: 32e8eaf263d9 ("um: use execveat to create userspace MMs")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20241016-uml-fix-stub_exe-clang-v1-1-3d6381dc5a78@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>