Split off functionality that relies on libm

Split off functionality that relies on libm

Previously some parts of compiler-rt depended on functions provided by a
math library, commonly supplied together with a real libc.
This change sections off the parts dependent on libm, allowing a minimal
compiler-rt to be built with nolibc alone.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #18