Kbuild, lto: Add Link Time Optimization support v3

Kbuild, lto: Add Link Time Optimization support v3

With LTO gcc will do whole program optimizations for
the whole kernel and each module. This increases compile time,
but can generate faster and smaller code and allows
the compiler to do global checking. For example the compiler
can complain now about type mismatches for symbols between
different files.

LTO allows gcc to inline functions between different files and
do various other optimization across the whole binary.

It might also trigger bugs due to more aggressive optimizations.
It allows gcc to drop unused code. It also allows it to check
types over the whole program.

The compile time is definitely slower. For gcc 4.8 on a
typical monolithic config it is about 58% slower. 4.9
drastically improved performance, with slowdown being
38% or so. Also incremenential rebuilds are somewhat
slower, as the whole kernel always needs to be reoptimized.
Very modular kernels have less build time slow down, as
the LTO will run for each module individually.

This adds the basic Kbuild plumbing for LTO:

- In Kbuild add a new scripts/Makefile.lto that checks
the tool chain (note the checks may not be fully bulletproof)
and when the tests pass sets the LTO options
Currently LTO is very finicky about the tool chain.
- Add a new LDFINAL variable that controls the final link
for vmlinux or module. In this case we call gcc-ld instead
of ld, to run the LTO step.
- For slim LTO builds (object files containing no backup
executable) force AR to gcc-ar
- Theoretically LTO should pass through compiler options from
the compiler to the link step, but this doesn't work for all options.
So the Makefile sets most of these options manually.
- Kconfigs:
Since LTO with allyesconfig needs more than 4G of memory (~8G)
and has the potential to makes people's system swap to death.
I used a nested config that ensures that a simple
allyesconfig disables LTO. It has to be explicitely
enabled.
- Some depencies on other Kconfigs:
MODVERSIONS, GCOV, FUNCTION_TRACER, KALLSYMS_ALL, single chain WCHAN are
incompatible with LTO currently, mostly because they
they require setting special compiler options
for specific files, which LTO currently doesn't support.
MODVERSIONS should in principle work with gcc 4.9, but still disabled.
FUNCTION_TRACER/GCOV can be fixed with a unmerged gcc patch.
- Also disable strict copy user checks because they trigger
errors with LTO.
- modpost symbol checking is downgraded to a warning,
as in some cases modpost runs before the final link
and it cannot resolve LTO symbols at this point.

For more information see Documentation/lto-build

Thanks to HJ Lu, Joe Mario, Honza Hubicka, Richard Guenther,
Don Zickus, Changlong Xie who helped with this project
(and probably some more who I forgot, sorry)

v2:
Merge documentation file into this patch
Improve documentation and Kconfig, fix a lot of obsolete comments.
Exclude READABLE_ASM
Some random fixes
v3:
Remove CONFIG_LTO_SLIM, is on by default.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>