[AArch64] Increase default function alignment

[AArch64] Increase default function alignment

With -mcpu=generic the function alignment is currently 8, however almost all
supported cores prefer 16 or higher, so increase the default to 16:12.
This gives ~0.2% performance increase on SPECINT2017, while codesize is 0.12%
larger.

    gcc/
	* config/aarch64/aarch64.c (generic_tunings): Set function alignment to
	16:12.

From-SVN: r274374