MIPS64: Fix art_quick_aput_obj stubs

Prevent situations where $gp is calculated and saved twice in a row
without restoring it. When restored in the end, its value was wrong.

This fixes test failure in stub_test.APutObj.

Test: mma test-art-target-gtest-stub_test64 on QEMU

Change-Id: I63b37ecee8d3576c57c0f6862d00bc03965b042d