[AArch64] Do not increase data alignment at -Os and with -fconserve-stack.

[AArch64] Do not increase data alignment at -Os and with -fconserve-stack.

We unnecessarily align data to 8 byte alignments even when -Os is
specified. This brings the logic in the AArch64 backend more in line
with the ARM backend and helps gain some image size in a few
places. Caught by an internal report on the size of rodata sections
being high with aarch64 gcc.

* config/aarch64/aarch64.h (AARCH64_EXPAND_ALIGNMENT): New.
  (DATA_ALIGNMENT): Update to use AARCH64_EXPAND_ALIGNMENT.
  (LOCAL_ALIGNMENT): Update to use AARCH64_EXPAND_ALIGNMENT.

Bootstrapped and regression tested on aarch64-none-linux-gnu with no
regressions.

From-SVN: r249764