Add cflags sub-property to 'pgo'

This (possibly arch-specific) property adds extra flags while building
for profiling.  For example, dex2oat requires setting
'-Wno-frame-larger-than' when instrumentation is enabled.

Test: Build 'pgo' module with the new property and inspect build.ninja
Change-Id: Ia36422a400b0e0b87cbb33b1c8e565569475a56d