Remove hardcoded -O3.

Index: lib/x86_64/fast/CMakeLists.txt
--- lib/x86_64/fast/CMakeLists.txt.orig
+++ lib/x86_64/fast/CMakeLists.txt
@@ -40,5 +40,5 @@ endif()
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 set(SRCS
   fastmath.S)
-libmath_add_object_library("${SRCS}" "-m64 -O3 -mtune=skylake-avx512 -march=skylake-avx512" "${DEFINITIONS}" "fastmath")
+libmath_add_object_library("${SRCS}" "-m64 -mtune=skylake-avx512 -march=skylake-avx512" "${DEFINITIONS}" "fastmath")
 add_dependencies(fastmath mthdecls)
