platform: fix unaligned 64-bit accesses on AArch32 (#702)

Ensures that Aarch32 Arm builds with an Armv8 compiler do not set
BROTLI_64_BITS.

This scenario is possible with ChromeOS builds, as they may use a
toolchain with the target armv7-cros-gnueabi, but with -march=armv8.
This will set __ARM_ARCH to 8 (defining BROTLI_TARGET_ARMV8), but will
also set __ARM_32BIT_STATE and not __ARM_64BIT_STATE. Without this,
illegal 64-bit non-word-aligned reads (LDRD) may be emitted.

Also fix unaligned 64-bit reads on AArch32 - STRD was still possible to
emit.