Reduce space lost to object alignment

Currently, sections within .text/.rodata/.data/.bss are emitted in the
order they are seen by the linker. This leads to wasted space, when a
section with a larger alignment follows one with a smaller alignment.
We can avoid this wasted space by sorting the sections.

To take full advantage of this, we must disable generation of common
symbols, so "common" data can be sorted along with the rest of .bss.

An example of the improvement, from `make DEBUG=1 PLAT=sun50i_a64 bl31`:
  .text   => no change
  .rodata => 16 bytes saved
  .data   => 11 bytes saved
  .bss    => 576 bytes saved

As a side effect, the addition of `-fno-common` in TF_CFLAGS makes it
easier to spot bugs in header files.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: I073630a9b0b84e7302a7a500d4bb4b547be01d51