AArch64: Tune case-values-threshold

Tune the case-values-threshold setting for modern cores.  A value of 11 improves
SPECINT2017 by 0.2% and reduces codesize by 0.04%.  With -Os use value 8 which
reduces codesize by 0.07%.

2021-10-18  Wilco Dijkstra  <wdijkstr@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_case_values_threshold):
	Change to 8 with -Os, 11 otherwise.