gn build: Make check-{clang,lld,llvm} pass on FreeBSD.

[pcc]

Summary

Mostly achieved by assuming that anything that isn't Win or Mac is ELF,
which seems reasonable enough for now.

[thakis]

Nice!

[pcc]

With a few tweaks check-llvm passes as well. Still looks good?

[thakis]

Yup!

Kind of makes me wish I had already done the plugin() / plugin_host() from TODO.txt, but oh well.

llvm/utils/gn/secondary/clang/tools/libclang/BUILD.gn

 import("//clang/lib/ARCMigrate/enable.gni")
 import("//llvm/version.gni")
 
 # This build file is just enough to get check-clang to pass, it's missing
 # several things from the CMake build:
 # - linking in clangTidyPlugin and clangIncludeFixerPlugin from
 #   clang-tools-extra (which doesn't have any GN build files yet)
 # - using libclang.exports
 # - a build target copying the Python bindings
 # - the GN linux build always builds without -fPIC (as if LLVM_ENABLE_PIC=OFF
 #   in the CMake build), so libclang is always a static library on linux
 # - the GN build doesn't have LIBCLANG_BUILD_STATIC
 
 libclang_target_type = "shared_library"
-if (host_os == "linux") {
-  # Linux needs -fPIC to build shared libs but they aren't on by default.
+if (host_os != "win" && host_os != "mac") {
+  # ELF targets need -fPIC to build shared libs but they aren't on by default.
   # For now, make libclang a static lib there.
   libclang_target_type = "static_library"
 }
 
 target(libclang_target_type, "libclang") {
   configs += [ "//llvm/utils/gn/build:clang_code" ]
   deps = [
     "//clang/include/clang/Config",

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

values += [
       "HAVE_ZLIB_H=1",
       "HAVE__CHSIZE_S=",
       "HAVE__UNWIND_BACKTRACE=1",
       "stricmp=",
       "strdup=",
     ]
   }
 
-  if (current_os == "linux" || current_os == "android") {
-    values += [ "LTDL_SHLIB_EXT=.so" ]
-  } else if (current_os == "mac") {
+  if (current_os == "mac") {
     values += [ "LTDL_SHLIB_EXT=.dylib" ]
   } else if (current_os == "win") {
     values += [ "LTDL_SHLIB_EXT=.dll" ]
+  } else {
+    values += [ "LTDL_SHLIB_EXT=.so" ]
   }
 
   if (llvm_enable_libedit) {
     values += [ "HAVE_LIBEDIT=1" ]
   } else {
     values += [ "HAVE_LIBEDIT=" ]
   }
 

llvm/utils/gn/secondary/llvm/test/BUILD.gn

write_lit_config("lit_site_cfg") {
   ]
 
   if (host_cpu == "x64") {
     extra_values += [ "HOST_ARCH=x86_64" ]
   } else {
     assert(false, "unimplemented host_cpu " + host_cpu)
   }
 
+  if (host_os == "win") {
+    extra_values += [ "EXEEXT=.exe" ]
+  } else {
+    extra_values += [ "EXEEXT=" ]
+  }
+
   if (host_os == "mac") {
-    extra_values += [
-      "EXEEXT=",
-      "HOST_OS=Darwin",
-      "SHLIBEXT=.dylib",
-    ]
+    extra_values += [ "SHLIBEXT=.dylib" ]
+  } else if (host_os == "win") {
+    extra_values += [ "SHLIBEXT=.dll" ]
+  } else {
+    extra_values += [ "SHLIBEXT=.so" ]
+  }
+
+  if (host_os == "freebsd") {
+    extra_values += [ "HOST_OS=FreeBSD" ]
+  } else if (host_os == "mac") {
+    extra_values += [ "HOST_OS=Darwin" ]
   } else if (host_os == "linux") {
-    extra_values += [
-      "EXEEXT=",
-      "HOST_OS=Linux",
-      "SHLIBEXT=.so",
-    ]
+    extra_values += [ "HOST_OS=Linux" ]
   } else if (host_os == "win") {
-    extra_values += [
-      "EXEEXT=.exe",
-      "HOST_OS=Windows",
-      "SHLIBEXT=.dll",
-    ]
+    extra_values += [ "HOST_OS=Windows" ]
   } else {
     assert(false, "unsupported host_os " + host_os)
   }
   if (host_os == "linux") {
     # lit.cfg.py's have_ld_plugin_support() checks for "gold" in --version,
     # so just claim that ld is gold on Linux.  The function also checks if
     # LLVMgold.so exists, but since that target isn't hooked up yet in the GN
     # build the LLVMgold.so tests currently don't run anywhere in the GN build.

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

 if (current_cpu == "x64") {
-  if (current_os == "linux") {
+  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"
   }
 } else if (current_cpu == "arm64") {
   if (current_os == "android") {