Disable call to fma for soft-float

PowerPC backend generate calls to libc function calls
for soft-float, regardless of the -nostdlib /-ffreestanding flag.
fma is not a function provided by compiler-rt builtins and
thus should not be generated here.
PR : [[ https://github.com/llvm/llvm-project/issues/55230 | #55230 ]]

Below is patch given by @nemanjai

Reviewed By: jhibbits

Differential Revision: https://reviews.llvm.org/D156344