ARM: compressed/head.S: work around new binutils warning

In August 2012, Matthew Gretton-Dann checked a change
into binutils labelled "Error on obsolete & warn on
deprecated registers", apparently as part of ARMv8
support. Apparently, this was supposed to emit
the message "Warning: This coprocessor register access
is deprecated in ARMv8" when using certain mcr/mrc
instructions and building for ARMv8. Unfortunately,
the message that is actually emitted appears to be
'(null)', which is less helpful in comparison.

Even more unfortunately, this is biting us on
every single kernel build with a new gas, because
arch/arm/boot/compressed/head.S and some other files
in that directory are built with -march=all since
kernel commit 80cec14a8 "[ARM] Add -march=all to
assembly file build in arch/arm/boot/compressed"
back in v2.6.28.

This patch reverts Russell's nice solution and
replaces it with a more complex one that sprinkles
.arch statements inside of the head.S file in
functions that are executed in different architecture
levels, which seems to solve the original problem
just as well, and gets rid of the new one, too.

Without this patch, building anything results in:

arch/arm/boot/compressed/head.S: Assembler messages:
arch/arm/boot/compressed/head.S:565: Warning: (null)
arch/arm/boot/compressed/head.S:676: Warning: (null)
arch/arm/boot/compressed/head.S:698: Warning: (null)
arch/arm/boot/compressed/head.S:722: Warning: (null)
arch/arm/boot/compressed/head.S:726: Warning: (null)
arch/arm/boot/compressed/head.S:957: Warning: (null)
arch/arm/boot/compressed/head.S:996: Warning: (null)
arch/arm/boot/compressed/head.S:997: Warning: (null)
arch/arm/boot/compressed/head.S:1027: Warning: (null)
arch/arm/boot/compressed/head.S:1035: Warning: (null)
arch/arm/boot/compressed/head.S:1046: Warning: (null)
arch/arm/boot/compressed/head.S:1060: Warning: (null)
arch/arm/boot/compressed/head.S:1092: Warning: (null)
arch/arm/boot/compressed/head.S:1094: Warning: (null)
arch/arm/boot/compressed/head.S:1095: Warning: (null)
arch/arm/boot/compressed/head.S:1102: Warning: (null)
arch/arm/boot/compressed/head.S:1134: Warning: (null)

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Gretton-Dann <matthew.gretton-dann@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Paul Beeler <sparksco@gmail.com>
Turn off -Wmaybe-uninitialized when building with -Os

gcc-4.7 and higher add a lot of false positive warnings about
potential uses of uninitialized warnings, but only when optimizing
for size (-Os). This is the default when building allyesconfig,
which turns on CONFIG_CC_OPTIMIZE_FOR_SIZE.

In order to avoid getting a lot of patches that initialize such
variables and accidentally hide real errors along the way, let's
just turn off this warning on the respective gcc versions
when building with size optimizations. The -Wmaybe-uninitialized
option was introduced in the same gcc version (4.7) that is now
causing the false positives, so there is no effect on older compilers.

A side effect is that when building with CONFIG_CC_OPTIMIZE_FOR_SIZE,
we might now see /fewer/ warnings about possibly uninitialized
warnings than with -O2, but that is still much better than seeing
warnings known to be bogus.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Git-commit: e74fc973b6e531fef1fce8b101ffff05ecfb774c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
return_address.c: Fix warning - TODO: return_address should use unwind tables