libkern: Add libcalls for ffs and __ffsdi2

These are needed when compiling a RISC-V kernel with GCC which does
not inline __builtin_ffs*.

The __ffsdi2 is adapated from the previous ffsl.c.

This pa

libkern: Add libcalls for ffs and __ffsdi2

These are needed when compiling a RISC-V kernel with GCC which does
not inline __builtin_ffs*.

The __ffsdi2 is adapated from the previous ffsl.c.

This partially reverts commit f4db342d44198973c1c7b9005d0c5683a582707e.

Reviewed by: mhorne
Differential Revision: https://reviews.freebsd.org/D54967

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