MIPS64: Make GetFeatureString() returns default instead of empty string

Since there isn't any feature left for MIPS64 (smp was recently
removed), GetFeatureString() returns an empty string. This causes
failures when running run-tests. dex2oat fails with following message:

Error parsing '--instruction-set-features=': No instruction set
features specified

Also change expectations in instruction_set_features_mips64_test.

Test: mma test-art-host-gtest
Test: mma test-art-target-gtest-instruction_set_features_mips64_test

Change-Id: Icc60a676a1b2fb10f89c888fa8a0d21fd0a105b5