libgcc: apply LIB2FUNCS_EXCLUDE logic to LIB2FUNCS_ST

libgcc: apply LIB2FUNCS_EXCLUDE logic to LIB2FUNCS_ST

One target file (config/c6x/t-elf) lists _printf and _gcc_bcmp in
LIB2FUNCS_EXCLUDE, but that does not have any effect, since those are
not filtered away from LIB2FUNCS_ST. Another option is to do as in
config/rl78/t-rl78, which explicitly sets LIB2FUNCS_ST

# Remove __gcc_bcmp from LIB2FUNCS_ST
LIB2FUNCS_ST = _eprintf

but honouring LIB2FUNCS_EXCLUDE also for LIB2FUNCS_ST seems more
natural.

From-SVN: r265246