Fix some FP operations on x86 targets under -opt

#KT-37412 Fixed

backend.native/tests/codegen/arithmetic/basic.kt

@@ -87,4 +87,10 @@ fun integralToFloat() {
 fun compareIntToFloat() {
     assertEquals(1, 0.compareTo(-0.0f))
     assertEquals(0, 0.compareTo(+0.0f))
+}
+
+@Test
+fun testKt37412() {
+    val two = 2.0
+    assertEquals(2, two.toInt())
 }

konan/konan.properties

@@ -277,7 +277,11 @@ target-sysroot-xcode_11_4-watchos_x86.default = \
 arch.watchos_x86 = i386
 targetSysRoot.watchos_x86 = target-sysroot-xcode_11_4-watchos_x86
 
-clangFlags.watchos_x86 = -cc1 -emit-obj -disable-llvm-passes -x ir
+# -target-cpu pentium4 makes sure that cogenerator knows which CPU flavour to emit code for.
+# Can be seen on optimized build and FP tests, where value is passed on stack
+# by Kotlin and taken from SSE registers by C code.
+# TODO: once this information is available in function attributes, we can remove this flag.
+clangFlags.watchos_x86 = -cc1 -emit-obj -disable-llvm-passes -x ir -target-cpu pentium4
 clangNooptFlags.watchos_x86 = -O1
 clangOptFlags.watchos_x86 = -O3
 clangDebugFlags.watchos_x86 = -O0
@@ -549,7 +553,12 @@ dependencies.mingw_x64-android_x86 = \
     libffi-3.2.1-mingw-w64-x86-64
 
 quadruple.android_x86 = i686-linux-android
-clangFlags.android_x86 = -cc1 -target-cpu i686 -emit-obj -disable-llvm-passes -x ir -femulated-tls -mrelocation-model pic
+
+# -target-cpu pentium4 makes sure that cogenerator knows which CPU flavour to emit code for.
+# Can be seen on optimized build and FP tests, where value is passed on stack
+# by Kotlin and taken from SSE registers by C code.
+# TODO: once this information is available in function attributes, we can set target CPU to i686.
+clangFlags.android_x86 = -cc1 -target-cpu pentium4 -emit-obj -disable-llvm-passes -x ir -femulated-tls -mrelocation-model pic
 clangOptFlags.android_x86 = -O3 -ffunction-sections
 clangNooptFlags.android_x86 = -O1
 targetSysRoot.android_x86 = target-sysroot-1-android_ndk