gcc: prevent the use of LDRD/STRD on ARMv5TE

These instructions are for 64-bit load/store. On ARMv5TE, the CPU
requires addresses to be aligned to 64-bit. When misaligned, behavior is
undefined (effectively either loads the same word twice on LDRD, or
corrupts surrounding memory on STRD).

On ARMv6 and newer, unaligned access is safe.

Removing these instructions for ARMv5TE is necessary, because GCC
ignores alignment information in pointers and does unsafe optimizations
that have shown up as bugs in various places.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 39638