retpoline/module: Warn for missing retpoline in module

There's a risk that a kernel that has full retpoline mitigations
becomes vulnerable when a module gets loaded that hasn't been
compiled with the right compiler or the right option.

We cannot fix it, but should at least warn the user when that
happens.

When the a module hasn't been compiled with a retpoline
aware compiler, print a warning and change the SPECTRE_V2
mitigation mode to show the system is vulnerable now.

For modules it is checked at compile time, however it cannot
check assembler or other non compiled objects used in the module link.

v2: Change warning message
v3: Port to latest tree
v4: Remove tainting
Cc: jeyu@kernel.org
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>