rtld-elf: link udivmoddi4 from compiler_rt

This fixes the gcc9 build of rtld-elf32 on amd64, which needed an
implementation of udivmoddi4.

rtld-elf uses certain functions normally found in libc, and so it
includes certain files from libc in its own build. It has two
mechanisms to include files from libc: one that rebuilds source files in
the rtld-elf environment, and one that extracts object files from a
purpose-built no-SSP PIC archive.

In addition to libc functions, rtld-elf may need to link functions
normally found in libcompiler_rt (formerly libgcc). Now, add an ability
to rebuild libcompiler_rt source files in the rtld-elf environment. We
don't yet have a need for an object file extraction mechanism.

libcompiler_rt could also supply udivdi3 and umoddi3, but leave them
alone for now.

Reviewed by: arichardson, kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27665