MFC r351119, r351135-r351136, r351412: stand xtoolchain-llvm90 fixes

r351119:
stand: push LIBC_SRC up into defs.mk

Other parts of stand/ that don't use libsa will need to grab bits from libc
shortly. Push LIBC_SRC up to defs.mk in advance of this so that they can use
it, and rename it to LIBCSRC to match the convention of the rest of the *SRC
variables in this file.

r351135:
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.

r351136:
stand: gptboot: fix build with xtoolchain-llvm90

ufsread.c grows a dependency on __ashldi3 with llvm90. For gptboot, just
start pulling in ashldi3.c ashrdi3.c lshrdi3.c into libsa for all archs as
the number of archs requiring one or more of them keeps growing. qdivrem.c
and quad.h can be trivially kicked out of libsa if we start pulling these
from compiler-rt as qdivrem was only used to implement umoddi3, divdi3,
moddi3 (also in qdivrem.c).

r351412:
stand: boot2: fix amd64-xtoolchain-gcc build

-Wno-missing-declarations is the GCC equivalent of
-Wno-missing-prototypes... this was overlooked in r351135.