sys: ${CFLAGS:N-flto} -> ${CFLAGS:N-flto*}

For the same reason as the original https://reviews.freebsd.org/D9659:
-flto=<N>, -flto=full, and -flto=thin also produce the GIMPLE/bitcode
which is not supported by genassym, so filter those out as well.

Signed-off-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/898