arm: Have the linker garbage-collect unused functions and variables

This patch activates -ffunction-sections and -fdata-sections for the
compiler and --gc-sections for the linker. This will strip out all
unused functions and static/global variables from the final binaries and
reduce the amount of data we need to read over SPI.

A quick test with ToT images shows a 2.5k (13%) / 10k (29%) / 12k (28%)
reduction on Nyan and 3k (38%) / 23k (50%) / 13k (29%) on Pit,
respectively for bootblock / romstage / ramstage.

Change-Id: I052411d4ad190d0395921ac4d4677341fb91568a
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/177111
(cherry picked from commit 5635b138778dea67a5f179e13003132be07f7e59)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6904
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

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