MIPS32: Remove memcpy() from java.lang.String.getChars intrinsic.

Test: run-test --optimizing 020-string
Test: run-test 020-string
Test: run-test --no-prebuild --optimizing 020-string
Test: run-test --no-prebuild 020-string
Test: mma -j2 ART_TEST_OPTIMIZING=true test-art-target-run-test
Test: booted MIPS32R2 emulator.

Note: Tested against both the MIPS32R2, and MIPS64R6 emulators.

Change-Id: I3219bc47a3657b82c82b6ac94d0b8c82eb6f9bb9