The one new assembly source file, `arm/adddf3.S`, implements both
addition and subtraction via cross-branching after flipping signs, since
both operations must provide substantially the same logic. The new cmake
properties introduced in a prior commit are used to arrange that
including `adddf3.S` supersedes the C versions of both addition and
subtraction, and also informs the test suite that both functions are
available to test.
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