ART: Mark StringBuilder.append()s as intrinsics.

We do not actually emit intrinsic implementations for these
methods but we use this designation to simplify their use
knowing that they "return this". This also prepares for
recognizing and optimizing the StringBuilder append pattern.

Note that intrinsics are never inlined but these methods are
defined in core-oj and therefore the rest of the boot image
and prebuilts are not allowed to inline them anyway.

Boot image sizes for aosp_taimen-userdebug:
 - before:
   arm/boot*.oat: 19756400
   arm64/boot*.oat: 23415656
 - after:
   arm/boot*.oat: 19653872 (-100KiB)
   arm64/boot*.oat: 23292784 (-120KiB)

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: aosp_taimen-userdebug boots
Test: run-gtests.sh
Test: testrunner.py --target --optimizing
Bug: 19575890
Change-Id: I2f3bac17830ac1c51287b090a96facddd3d09f5c
13 files changed