Provide support for ARMv4, lacking bx and clz. Unroll the
test-and-subtract loop and compute the initial block as address,
shaving off between 5% and 10% on Cortex A9 and 30%+ a Raspberry Pi.
Code written by Matt Thomas and Joerg Sonnenberger.

Differential Revision: http://llvm-reviews.chandlerc.com/D2595


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200001 91177308-0d34-0410-b5e6-96231b3b80d8