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>