-mk/bsd.lib.mk picks up a .S asm file behind our back (did it do so
always?). s_modf.S is incorrect; I'm undecided whether it makes sense
to fix it, so add a stupid rule to enforce the .c file to be used.
-on i386, s_tanh.c gets miscompiled. It works with -O0, so add
COPTS for exactly that file.
2dcdd0a999