Reduce libm's exports and make internal calls go direct.
All dependencies on libc are now via reserved/standardized names.

ok kettenis@ millert@ deraadt@

Move llrint.c, llrintf.c, lrint.c and lrintf.c to s_llrint.c, s_llrintf.c,
s_lrint.c and s_lrintf.c, and add i387-specific assembler versions.

ok deraadt@, otto@