gn build: Add support for building ubsan_minimal.

[thakis]

nit: llvm/utils/gn/gn.py format llvm/utils/gn/secondary/compiler-rt/lib/ubsan_minimal/BUILD.gn (and if that doesn't change anything, then llvm/utils/gn/get.py and then try again

[thakis]

This broke the build on macs: http://45.33.8.238/macm1/12180/step_4.txt I moved this to linux-only for now in e8c8ce0974edca7bc21ce53826ff7b2c0456d70a

Please watch http://45.33.8.238/ for ~15 min after landing gn changes.

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

 group("lib") {
   deps = [ "//compiler-rt/lib/profile" ]
   if (current_os == "linux") {
     deps += [ "//compiler-rt/lib/msan" ]
   }
   if (current_os != "win") {
     deps += [
       "//compiler-rt/lib/asan",
       "//compiler-rt/lib/builtins",
       "//compiler-rt/lib/tsan",
+      "//compiler-rt/lib/ubsan_minimal",
     ]
   }
 }

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

+import("//compiler-rt/target.gni")
+
+source_set("sources") {
+  configs -= [ "//llvm/utils/gn/build:llvm_code" ]
+  configs += [ "//llvm/utils/gn/build:crt_code" ]
+}
+
+static_library("ubsan_minimal") {
+  output_dir = crt_current_out_dir
+  output_name = "clang_rt.ubsan_minimal$crt_current_target_suffix"
+  complete_static_lib = true
+  configs -= [
+    "//llvm/utils/gn/build:llvm_code",
+    "//llvm/utils/gn/build:thin_archive",
+  ]
+  configs += [ "//llvm/utils/gn/build:crt_code" ]
+  sources = [ "ubsan_minimal_handlers.cpp" ]
+}