linux: use INSTALL_MOD_STRIP=1 to strip modules

linux: use INSTALL_MOD_STRIP=1 to strip modules

We used to do a special handling of Linux kernel modules when stripping
target binaries because there's some special precious data in modules
that we must keep for them to properly operate. This is for example true
for stack unwinding data etc.

It turned out there're cases when our existing "strip --strip-unneeded"
doesn't work well. For example this removes .debug_frame section used by
Linux on ARC for stack unwinding, refer to [1] and [2] for more details.

Now Linux kernel may strip modules as a part of "modules_install" target
if INSTALL_MOD_STRIP=1 is passed in command line. And so we'll do
allowing kernel decide how to strip modules in the best way.

Still note as of today Linux kernel strips modules uniformly for all
arches with "strip" command, so this commit alone doesn't solve
mentioned problem but it opens a possibility to add later a patch to the
kernel which will strip modules for ARC differently - and that's our
plan for mainline kernel.

[1] https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/issues/86
[2] http://lists.busybox.net/pipermail/buildroot/2016-September/172161.html

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Daniel Mentz <danielmentz@google.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>