Fix math functions optimization

Replace fast-math with fp-contract=fast

Issue:#I66OIA
Signed-off-by: Nguzhihao4 <guzhihao4@huawei.com>
Change-Id: I97f546b03ace00bcdac9932676a5c31a8e642d5c

musl_template.gni

@@ -601,7 +601,7 @@ template("musl_libs") {
     cflags = [
       "-mllvm",
       "-instcombine-max-iterations=0",
-      "-ffast-math",
+      "-ffp-contract=fast",
       "-O3",
       "-fPIC",
       "-fstack-protector-strong",