compiler-rt & LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON: using "incorrect" triple leads to a cryptic error message

[sylvestre]

Building llvm 18 on i386 with the option LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON & -DLLVM_HOST_TRIPLE=i386-linux-gnu failed with:

Call Stack (most recent call first):
  /<<PKGBUILDDIR>>/compiler-rt/lib/CMakeLists.txt:4 (include)


CMake Error at /<<PKGBUILDDIR>>/compiler-rt/cmake/Modules/AddCompilerRT.cmake:281 (message):
  Cannot find builtins library for the target architecture
Call Stack (most recent call first):
  /<<PKGBUILDDIR>>/compiler-rt/lib/stats/CMakeLists.txt:23 (add_compiler_rt_runtime)

Full log:
https://buildd.debian.org/status/fetch.php?pkg=llvm-toolchain-18&arch=i386&ver=1%3A18.1.8-2%7Eexp1&stamp=1720099326&raw=0

Changing from
-DLLVM_HOST_TRIPLE=i386-linux-gnu
to
-DLLVM_HOST_TRIPLE=i386-pc-linux-gnu
fixed the problem.
(see https://buildd.debian.org/status/fetch.php?pkg=llvm-toolchain-18&arch=i386&ver=1%3A18.1.8-2%7Eexp2&stamp=1720353987&raw=0 )

Dunno if it is an actual bug or we should just improve the error message.

cc @MaskRay @petrhosek