[gn build] Add support for Win/x86 compiler-rt

[aeubanks]

Summary

This allows us to build the x86 profile runtime.

[rnk]

lgtm, since @thakis is OOO, but I am not a gn expert.

llvm/utils/gn/build/toolchain/BUILD.gn

toolchain_args = {
     clang_base_path = root_build_dir
     use_goma = false
   }
   deps = [
     "//:clang($host_toolchain)",
     "//:lld($host_toolchain)",
   ]
 }
+
+win_toolchain("stage2_win_x86") {
+  toolchain_args = {
+    current_os = host_os
+    current_cpu = "x86"
+
+    clang_base_path = root_build_dir
+    use_goma = false
+  }
+  deps = [
+    "//:clang($host_toolchain)",
+    "//:lld($host_toolchain)",
+  ]
+}

llvm/utils/gn/secondary/compiler-rt/BUILD.gn

 import("//llvm/lib/Target/targets.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 # In the GN build, compiler-rt is always built by just-built clang and lld.
 # FIXME: For macOS and iOS builds, depend on lib in all needed target arch
 # toolchains and then lipo them together for the final output.
 if (current_os == "win") {
   supported_toolchains = [ "//llvm/utils/gn/build/toolchain:stage2_win" ]
+  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_win_x86" ]
 } else {
   supported_toolchains = [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
 }
 if (android_ndk_path != "") {
   supported_toolchains += [
     "//llvm/utils/gn/build/toolchain:stage2_android_aarch64",
     "//llvm/utils/gn/build/toolchain:stage2_android_arm",
   ]

llvm/utils/gn/secondary/llvm/triples.gni

 if (current_cpu == "x86") {
   if (current_os == "linux") {
     llvm_current_triple = "i386-unknown-linux-gnu"
+  } else if (current_os == "win") {
+    llvm_current_triple = "i386-pc-windows-msvc"
   }
 } else if (current_cpu == "x64") {
   if (current_os == "freebsd") {
     llvm_current_triple = "x86_64-unknown-freebsd"
   } else if (current_os == "linux") {
     llvm_current_triple = "x86_64-unknown-linux-gnu"
   } else if (current_os == "ios" || current_os == "mac") {
     llvm_current_triple = "x86_64-apple-darwin"