[compiler-rt][ARM] Optimized mulsf3 and divsf3 (#168394)
(Reland of #161546, fixing three build and test issues)
This commit adds optimized assembly versions of single-precision float
multiplication and division. Both functions are implemented in a style
that can be assembled as either of Arm and Thumb2; for multiplication, a
separate implementation is provided for Thumb1. Also, extensive new
tests are added for multiplication and division.
These implementations can be removed from the build by defining the
cmake variable COMPILER_RT_ARM_OPTIMIZED_FP=OFF.
Outlying parts of the functionality which are not on the fast path, such
as NaN handling and underflow, are handled in helper functions written
in C. These can be shared between the Arm/Thumb2 and Thumb1
implementations, and also reused by other optimized assembly functions
we hope to add in future.
