stand: boot2: fix build with xtoolchain-llvm90

ufsread.c grows a dependency on __ashldi3 with llvm90. Grab ashldi3.c out of
compiler-rt rather than trying to link against libsa (for now).
-Wno-missing-prototypes is necessary to compile ashldi3.c standalone.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21291