Switch power to use O3 instead of Ofast

Ofast enables possibly unsafe optimizations in addition to O3. This
appears to have been added and then just continually copied into later
Power architectures, and it wasn't included in the CMake build system
when that was introduced.

Replace this with O3 so that the same level of optimization is done by
the compiler.

Makefile.power

@@ -13,16 +13,16 @@ ifeq ($(CORE), POWER10)
 ifneq ($(C_COMPILER), PGI)
 ifeq ($(C_COMPILER), GCC)
 ifeq ($(GCCVERSIONGTEQ10), 1)
-CCOMMON_OPT += -Ofast -mcpu=power10 -mtune=power10 -mvsx -fno-fast-math
+CCOMMON_OPT += -O3 -mcpu=power10 -mtune=power10 -mvsx -fno-fast-math
 else ifneq ($(GCCVERSIONGT4), 1)
 $(warning your compiler is too old to fully support POWER9, getting a newer version of gcc is recommended)
-CCOMMON_OPT += -Ofast -mcpu=power8 -mtune=power8 -mvsx -fno-fast-math
+CCOMMON_OPT += -O3 -mcpu=power8 -mtune=power8 -mvsx -fno-fast-math
 else
 $(warning your compiler is too old to fully support POWER10, getting a newer version of gcc is recommended)
-CCOMMON_OPT += -Ofast -mcpu=power9 -mtune=power9 -mvsx -fno-fast-math
+CCOMMON_OPT += -O3 -mcpu=power9 -mtune=power9 -mvsx -fno-fast-math
 endif
 else
-CCOMMON_OPT += -Ofast -mcpu=power10 -mtune=power10 -mvsx -fno-fast-math
+CCOMMON_OPT += -O3 -mcpu=power10 -mtune=power10 -mvsx -fno-fast-math
 endif
 ifeq ($(F_COMPILER), IBM)
 FCOMMON_OPT += -O2 -qrecur -qnosave -qarch=pwr10 -qtune=pwr10 -qfloat=nomaf -qzerosize
@@ -34,7 +34,7 @@ endif
 
 ifeq ($(CORE), POWER9)
 ifneq ($(C_COMPILER), PGI)
-CCOMMON_OPT += -Ofast -mvsx -fno-fast-math
+CCOMMON_OPT += -O3 -mvsx -fno-fast-math
 ifeq ($(C_COMPILER), GCC)
 ifneq ($(GCCVERSIONGT4), 1)
 $(warning your compiler is too old to fully support POWER9, getting a newer version of gcc is recommended)
@@ -70,7 +70,7 @@ endif
 
 ifeq ($(CORE), POWER8)
 ifneq ($(C_COMPILER), PGI)
-CCOMMON_OPT += -Ofast -mcpu=power8 -mtune=power8 -mvsx  -fno-fast-math
+CCOMMON_OPT += -O3 -mcpu=power8 -mtune=power8 -mvsx  -fno-fast-math
 else
 CCOMMON_OPT += -fast -Mvect=simd -Mcache_align
 endif