With BuildFlavour set to `perf-llvm`, GHCi would fail as soon as it
was run with:

  ghc-stage2: .../ghc-prim/dist-install/build/HSghc-prim-0.5.0.0.o:
  unknown symbol `__udivti3'
  ghc-stage2: unable to load package `ghc-prim-0.5.0.0'

Fix this by adding `__udivti3` and `__umodti3` to RtsSymbols.c.

Test Plan: Validate

Reviewers: simonmar, austin, bgamari, Phyx, trofi

Reviewed By: Phyx, trofi

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2449

GHC Trac Issues: #11981