MIPS32: java.lang.String.getChars

Use memcpy(3) to copy characters under the assumption that memcpy()
has been hand optimized for best performance on the platform being
tested.

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: run-test --optimizing 082-inline-execute
Test: run-test 082-inline-execute
Test: run-test --no-prebuild --optimizing 082-inline-execute
Test: run-test --no-prebuild 082-inline-execute
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: I4192cf6244db120c8de5cc4932d4132acfc9740d