nios2: use gc sections to reduce image size

Follow the discussion of Charles Manning and Mike Frysinger.
Using gc_sections helps reduce image size.

Configuring for nios2-generic board...
Before,
   text    data     bss     dec     hex filename
 123979    3724   22892  150595   24c43 /tmp/u-boot/u-boot
After,
   text    data     bss     dec     hex filename
 115983    3800   22732  142515   22cb3 /tmp/u-boot/u-boot

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>

arch/nios2/config.mk

@@ -30,3 +30,6 @@ PLATFORM_CPPFLAGS += -DCONFIG_NIOS2 -D__NIOS2__
 PLATFORM_CPPFLAGS += -G0
 
 LDSCRIPT ?= $(SRCTREE)/$(CPUDIR)/u-boot.lds
+
+LDFLAGS += --gc-sections
+PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections