check for ld support of -Bsymbolic-functions; disable shared if not avail

check for ld support of -Bsymbolic-functions; disable shared if not avail

this issue affects the last gpl2 version of binutils, which some
people are still using out of aversion to gpl3. musl requires
-Bsymbolic-functions because it's the only way to make a libc.so
that's able to operate prior to dynamic linking but that still behaves
correctly with respect to global vars that may be moved to the main
program via copy relocations.