change Toolchain.get_flag so it doesn't automatically prepend a dash (-) to compiler flags, add deprecation warning for optarch value without leading dash, rename Compiler.COMPILER*_FLAGS to Compiler.COMPILER*_OPTIONS

change Toolchain.get_flag so it doesn't automatically prepend a dash (-) to compiler flags, add deprecation warning for optarch value without leading dash, rename Compiler.COMPILER*_FLAGS to Compiler.COMPILER*_OPTIONS #1897