gn build: Add 32-bit Linux support.

[thakis]

y tho

[pcc]

In D58839#1416172, @thakis wrote:

y tho

Main reason is that we have a 32-bit bot (well, ARM 32-bit, but x86 32-bit is close enough) and it's occasionally useful to be able to test changes on 32-bit (e.g. D58840).

llvm/utils/gn/build/toolchain/target_flags.gni

 import("//llvm/triples.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 target_flags = []
 target_ldflags = []
 
 if (current_os == "android") {
   target_flags += [
     "--target=$llvm_current_triple",
     "--sysroot=$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64/sysroot",
     "-B$android_ndk_path/toolchains/llvm/prebuilt/linux-x86_64",
   ]
   target_ldflags += [ "-static-libstdc++" ]
 }
+
+if (current_cpu == "x86") {
+  target_flags = [ "-m32" ]
+}

llvm/utils/gn/secondary/compiler-rt/target.gni

 import("//clang/resource_dir.gni")
 
 if (current_os == "linux" || current_os == "android") {
   crt_current_out_dir = "$clang_resource_dir/lib/linux"
 } else {
   assert(false, "unimplemented current_os " + current_os)
 }
 
-if (current_cpu == "x64") {
+if (current_cpu == "x86") {
+  crt_current_target_arch = "i386"
+} else if (current_cpu == "x64") {
   crt_current_target_arch = "x86_64"
 } else if (current_cpu == "arm64") {
   crt_current_target_arch = "aarch64"
 } else {
   assert(false, "unimplemented current_cpu " + current_cpu)
 }
 
 crt_current_target = crt_current_target_arch
 if (current_os == "android") {
   crt_current_target += "-android"
 }

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

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