MIPS64: Remove memcpy() from java.lang.String.getChars intrinsic.

Test: run-test --64 --optimizing 020-string
Test: run-test --64 020-string
Test: run-test --64 --no-prebuild --optimizing 020-string
Test: run-test --64 --no-prebuild 020-string
Test: mma -j2 ART_TEST_OPTIMIZING=true test-art-target-run-test
Test: mma test-art-target-gtest -j2
Test: booted MIPS64R6 emulator.

Note: All tests run against MIPS64 QEMU.

Change-Id: I8ad3e39c66a489779476f7e9f9f88cd0c6270c20