Map -O to -O1 instead of -O2

rL82131 changed -O from -O1 to -O2, because -O1 was not different from
-O2 at that time.

GCC treats -O as -O1 and there is now work to make -O1 meaningful.
We can change -O back to -O1 again.

Reviewed By: echristo, dexonsmith, arphaman

Differential Revision: https://reviews.llvm.org/D79916