[Fuchsia] Build compiler-rt builtins for 32-bit x86

[phosek]

Summary

While we don't support 32-bit architectures in Fuchsia, these are needed
in the early boot phase on x86, so we build just these to satisfy that
use case.

[mcgrathr]

lgtm

clang/cmake/caches/Fuchsia-stage2.cmake

if(LINUX_${target}_SYSROOT)
 
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endif()
 endforeach()
 
 if(FUCHSIA_SDK)
   set(FUCHSIA_aarch64_NAME arm64)
+  set(FUCHSIA_i386_NAME x64)
   set(FUCHSIA_x86_64_NAME x64)
   set(FUCHSIA_riscv64_NAME riscv64)
-  foreach(target x86_64;aarch64;riscv64)
+  foreach(target i386;x86_64;aarch64;riscv64)
     set(FUCHSIA_${target}_COMPILER_FLAGS "-I${FUCHSIA_SDK}/pkg/fdio/include")
     set(FUCHSIA_${target}_LINKER_FLAGS "-L${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/lib")
     set(FUCHSIA_${target}_SYSROOT "${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/sysroot")
   endforeach()
 
-  foreach(target x86_64;aarch64;riscv64)
+  foreach(target i386;x86_64;aarch64;riscv64)
     # Set the per-target builtins options.
     list(APPEND BUILTIN_TARGETS "${target}-unknown-fuchsia")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_ASM_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_C_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_CXX_FLAGS ${FUCHSIA_${target}_COMPILER_FLAGS} CACHE STRING "")
     set(BUILTINS_${target}-unknown-fuchsia_CMAKE_SHARED_LINKER_FLAGS ${FUCHSIA_${target}_LINKER_FLAGS} CACHE STRING "")

clang/lib/Basic/Targets.cpp

case llvm::Triple::x86:
     case llvm::Triple::DragonFly:
       return new DragonFlyBSDTargetInfo<X86_32TargetInfo>(Triple, Opts);
     case llvm::Triple::NetBSD:
       return new NetBSDI386TargetInfo(Triple, Opts);
     case llvm::Triple::OpenBSD:
       return new OpenBSDI386TargetInfo(Triple, Opts);
     case llvm::Triple::FreeBSD:
       return new FreeBSDTargetInfo<X86_32TargetInfo>(Triple, Opts);
+    case llvm::Triple::Fuchsia:
+      return new FuchsiaTargetInfo<X86_32TargetInfo>(Triple, Opts);
     case llvm::Triple::KFreeBSD:
       return new KFreeBSDTargetInfo<X86_32TargetInfo>(Triple, Opts);
     case llvm::Triple::Minix:
       return new MinixTargetInfo<X86_32TargetInfo>(Triple, Opts);
     case llvm::Triple::Solaris:
       return new SolarisTargetInfo<X86_32TargetInfo>(Triple, Opts);
     case llvm::Triple::Win32: {
       switch (Triple.getEnvironment()) {