[ARM] Tweak HONOR_REG_ALLOC_ORDER

[ARM] Tweak HONOR_REG_ALLOC_ORDER

Setting HONOR_REG_ALLOC_ORDER improves codesize with -Os, however it generates
slower and larger code with -O2 and higher.  So only set it when optimizing for
size.  On Cortex-A57 this improves SPECINT2006 by 0.15% and SPECFP2006 by 0.25%
while reducing codesize.

    gcc/
	* config/arm/arm.h (HONOR_REG_ALLOC_ORDER): Set when optimizing for
	size.

From-SVN: r276887