Picolibc needs two additional gcc build options so that libstdc++
works correctly. When building picolibc as a companion library, those
are added in do_cc_libstdcxx_picolibc, but when built with picolibc as
the main C libary, those need to be added in the main GCC build.
Signed-off-by: Keith Packard <keithp@keithp.com>