gn-build: Add missing global compiler flags

gn-build: Add missing global compiler flags

This CL introduces compiler flags that enable stripping of unused symbols out of
static libraries at the linking stage. This is used by the Android build to keep
module dependencies small while building specialized executables (such as unit
tests).

Also added a preprocessor definition for the EXPORT_SYMBOL macro, recently
introduced by icoolidge@.

BUG=21339022

Change-Id: I7d3f9f64abc1f4e080196c18ccb893443ee1b64a