Re-enable boot image direct string loads for read barriers

Boot.oat code size with CC baker:
ARM32: 70775656 -> 69817028 (-1.35%)
ARM64: 80819424 -> 79417072 (-1.74%)
X86 unmeasured.
X86_64 unmeasured.

Performance unmeasured, should be faster.

Bug: 29516974

Test: test-art-host CC baker, N6P booting CC baker

Change-Id: I219faaca9ed17af81d2815fb5e124120f307af83