msba2: Use --gc-sections when linking.

 - Add `-Wl,--gc-sections` to LDFLAGS
 - Add `-fdata-sections` and `-ffunction-sections` to CFLAGS
 - Update lpc2387 linker script to keep the isr vector and init
   sections when garbage collecting.
 - Keep .ctors and .dtors sections (similar to what binutils default
   linker script does when building for Linux targets)

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>