[clang] Fix -fp-model={strict|precise} to disable -fapprox-func

`-fapprox-func` should be disabled by `-fp-model={strict|precise}`,
as well as other fast-math flags. See the last changes in
`clang/test/Driver/fp-model.c`.

Probably this route (`case options::OPT_ffp_model_EQ`) was forgot
to update in D106191 and D114564. There is no appropriate reason not
to disable the flag.

This commit also updates other regression tests, which are not directly
related to this bug, for consistency with other fast-math flags.

Differential Revision: https://reviews.llvm.org/D138109