RFC: always default to -mno-unaligned-access for bare-metal ARM

[Sandra Loosemore <sandra at codesourcery dot com>]

Recently I tracked down a target crash problem in an ARM EABI 
configuration running on a Cortex-A9 board to an unaligned access fault. 
 The startup code provided by the customer for this board doesn't 
enable the MMU, and unaligned access support requires the MMU to be 
enabled per

http://infocenter.arm.com/help/topic/com.arm.doc.faqs/ka13671.html

The GCC manual presently says "By default unaligned access is disabled 
for all pre-ARMv6 and all ARMv6-M architectures, and enabled for all 
other architectures."  I think it would be safer for GCC to default to 
-maligned-access only in configurations that are known also to enable 
the MMU, e.g. GNU/Linux.  Code being built for bare-metal ARM EABI 
targets typically includes low-level startup code and boot loaders that 
run before the MMU is enabled, and likewise I don't think RTEMS, 
uCLinux, etc imply MMU support, either.

We could tell all ARM EABI users to build with -mno-unaligned-access, 
but if multilibs for (say) -march=armv7-a are being provided then those 
must also be built with the "safe" setting to avoid crashes in e.g. the 
Newlib memcpy implementation.  I think it would be more user-friendly 
just to change the default.

ARM maintainers, WDYT?  I will work up and test a patch if there is 
agreement that this is a reasonable thing to do.

-Sandra