[llvm] Ensure that soft float targets don't emit `fma()` libcalls. (#106615)

The previous behavior could be harmful in some edge cases, such as
emitting a call to `fma()` in the `fma()` implementat

[llvm] Ensure that soft float targets don't emit `fma()` libcalls. (#106615)

The previous behavior could be harmful in some edge cases, such as
emitting a call to `fma()` in the `fma()` implementation itself.

Do this by just being more accurate in `isFMAFasterThanFMulAndFAdd()`.
This was already done for PowerPC; this commit just extends that to Arm,
z/Arch, and x86. MIPS and SPARC already got it right, but I added tests
for them too, for good measure.

Note: I don't have commit access.

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