musl: import floating point rounding and exceptions

I am interested in the symbol fesetround from libc, that allows a user to set
the rounding mode of the floating point unit. But let's be proactive and bring
them all in.

This is not present in musl for aarch64.

Reviewed-by: Claudio Fontana <claudio.fontana@huawei.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>