testsuite: Add case-values-threshold to pr107876.C

This test was failing on aarch64 because aarch64 overrides
TARGET_CASE_VALUES_THRESHOLD.  The maximum value that allows
the test to pass appears to be 6, but the default threshold
is 4 or 5 (depending on whether casesi is provided).
Going for 4 seemed safest, in case any target-specific
features force the maximum passing threshold to be lower
on some targets.

gcc/testsuite/
	* g++.dg/tree-ssa/pr107876.C: Add --param case-values-threshold=4.