Add -ffp-contract=off to .bazelrc.

This is needed for floating point stability.

The global placement depends on the output of the FFT library. Without
`-ffp-contract=off` the current FFT library produces different results
dependent on if the compiler is targeting an x86 architecture which has
the "FMA instruction".

The FMA instruction omits the rounding done in the multiplication
instruction, making the calculations produce different results for some
input values

See https://kristerw.github.io/2021/11/09/fp-contract/

See also OpenROAD change @
The-OpenROAD-Project/OpenROAD#4518

Signed-off-by: Tim Ansell <tansell@google.com>

.bazelrc

@@ -13,7 +13,9 @@
 # limitations under the License.
 
 build --cxxopt "-std=c++17"
+build --cxxopt "-ffp-contract=off"
 build --host_cxxopt "-std=c++17"
+build --host_cxxopt "-ffp-contract=off"
 build --crosstool_top=@llvm_toolchain//:toolchain
 
 build:ciremotebuild --crosstool_top=@llvm_toolchain//:toolchain