libc/csu: Support IFUNCs on riscv

When adding support to rtld-elf I neglected the fact that static
binaries can have IFUNCs. Add support for this too.

Fixes: 729d2b16b74f ("rtld-elf: Support IFUNCs on riscv")
MFC after: 1 week

(cherry picked from commit 1363acbf25de4c36e183cfa0b0e801d4dd9bf2ad)