Improve Thumb2 branch/load-literal fixup performance.

Replace per-Fixup dependents vectors with a single array
held by the assembler and referenced by the Fixups to avoid
the cost of many allocations with the default allocator.

This improves the compilation time of the boot.oat on N5,
AOSP ToT, by about ~3% as measured by the "Compile Time"
reported with --dump-timing (~2% of the "dex2oat took...").

Change-Id: I7121cdef32d9edc6d287e602d774ffe03f530d18