-fexpensive-optimizations is enabled by default at -O2
-fomit-frame-pointer is enabled at -O2 selectively by gcc depending upon
architecture if debug info is not hurt
-frename-registers - This might have some performance advantage on top
of O2 on architectures which have more registers and registers are left
after scheduling but it affects debuggability quite a bit so as a i
tradeoff we do not use it.
-feliminate-dwarf2-dups - We use this option to reduce the size of debug
information by removing duplicates this is only valid for dwarf2+ and we
use dwarf2 by default
-pipe uses buffers instead of temporary files internally it can speed
up compilation it has has issues with other assemblers but not
with GNU assembler and we use gas.
Separate out debug information related flags into a separate variable
DEBUG_FLAGS so distros can use/notuse them as they like
