treewide: add support for "gc-sections" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to
enable it treewide, where chosen packages can still opt-out via
"no-gc-sections".

Note: libnl, mbedtls and opkg only used the CFLAGS part without the
LDFLAGS counterpart. That doesn't help at all if the goal is to produce
smaller binaries. I consider that an accident, and this fixes it.

Note: there are also packages using only the LDFLAGS part. I didn't
touch those, as gc might have been disabled via CFLAGS intentionally.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/utils/e2fsprogs/Makefile

@@ -23,6 +23,7 @@ PKG_BUILD_DEPENDS:=util-linux e2fsprogs/host
 PKG_INSTALL:=1
 
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=gc-sections
 
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/host-build.mk
@@ -142,7 +143,7 @@ $(call Package/e2fsprogs)
   DEPENDS:= +e2fsprogs
 endef
 
-TARGET_CFLAGS += $(FPIC) -ffunction-sections -fdata-sections -flto
+TARGET_CFLAGS += $(FPIC) -flto
 
 TARGET_LDFLAGS += -flto
 
@@ -171,7 +172,6 @@ define Build/Compile
 		V=$(if $(findstring c,$(OPENWRT_VERBOSE)),1,) \
 		subst
 	+$(MAKE) $(PKG_JOBS) -C $(PKG_BUILD_DIR) \
-		LDFLAGS=-Wl,--gc-sections \
 		BUILDCC="$(HOSTCC)" \
 		DESTDIR="$(PKG_INSTALL_DIR)" \
 		ELF_OTHER_LIBS="$(TARGET_LDFLAGS) -luuid" \

package/utils/mdadm/Makefile

@@ -19,6 +19,7 @@ PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_CPE_ID:=cpe:/a:mdadm_project:mdadm
 
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=gc-sections
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -43,7 +44,6 @@ define Package/mdadm/conffiles
 endef
 
 TARGET_CFLAGS += \
-	-ffunction-sections -fdata-sections \
 	-DHAVE_STDINT_H -DNO_COROSYNC -DNO_DLM -DUSE_PTHREADS \
 	-DCONFFILE='\"/var/etc/mdadm.conf\"' \
 	-DMAP_DIR='\"/var/run/mdadm\"' \
@@ -53,8 +53,6 @@ TARGET_CFLAGS += \
 
 TARGET_CXFLAGS = -DNO_LIBUDEV
 
-TARGET_LDFLAGS += -Wl,--gc-sections
-
 MAKE_FLAGS += \
 		CHECK_RUN_DIR=0 \
 		CXFLAGS="$(TARGET_CXFLAGS)"

package/utils/mtd-utils/Makefile

@@ -19,6 +19,7 @@ PKG_INSTALL:=1
 PKG_FIXUP:=autoreconf
 
 PKG_FLAGS:=nonshared
+PKG_BUILD_FLAGS:=gc-sections
 
 PKG_BUILD_DEPENDS:=util-linux
 
@@ -64,9 +65,6 @@ CONFIGURE_ARGS += \
 	--without-zstd \
 	--without-lzo
 
-TARGET_CFLAGS += -ffunction-sections -fdata-sections
-TARGET_LDFLAGS += -Wl,--gc-sections
-
 define Package/ubi-utils/install
 	$(INSTALL_DIR) $(1)/usr/sbin
 	$(INSTALL_BIN) \