Use the new PO_CXXFLAGS from r202807 to remove the '-ffunction-sections'

flag when compiling profiled objects as it's ignored by the compiler.
This removes the associated warning for each file compiled.

MFC after:	1 month

gnu/lib/libstdc++/Makefile

@@ -21,6 +21,7 @@ CFLAGS+=	-I${GCCLIB}/include -I${SRCDIR}/include -I.
 CFLAGS+=	-frandom-seed=RepeatabilityConsideredGood
 CXXFLAGS+=	-fno-implicit-templates -ffunction-sections -fdata-sections \
 		-Wno-deprecated
+PO_CXXFLAGS=	${CXXFLAGS:N-ffunction-sections}
 
 DPADD=		${LIBM}
 LDADD=		-lm

gnu/lib/libsupc++/Makefile

@@ -23,6 +23,7 @@ CFLAGS+=	-I${GCCLIB}/include -I${SRCDIR} -I${GCCDIR}
 CFLAGS+=	-I${.CURDIR}/../libstdc++ -I.
 CFLAGS+=	-frandom-seed=RepeatabilityConsideredGood
 CXXFLAGS+=	-fno-implicit-templates -ffunction-sections -fdata-sections
+PO_CXXFLAGS=    ${CXXFLAGS:N-ffunction-sections}
 
 HDRS=	exception new typeinfo cxxabi.h exception_defines.h
 INCS=	${HDRS:S;^;${SRCDIR}/;}