[vm/linux] Don't expose symbols from statically-linked libc++.

Issue https://github.com/dart-lang/sdk/issues/38141

On Windows and OSX, we dynamically link against libc++, so there is no issue.
On Android we already hide symbols from libc++.

Change-Id: I17debc4d0efec3cebc203672333afb44390b0e0b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115403
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Samir Jindel <sjindel@google.com>

build/config/compiler/BUILD.gn

@@ -247,8 +247,6 @@ config("compiler") {
   # Linux-specific compiler flags setup.
   # ------------------------------------
   if (is_linux) {
-    cflags += [ "-pthread" ]
-    ldflags += [ "-pthread" ]
     if (is_clang) {
       if (current_cpu == "arm") {
         cflags += [ "--target=armv7-linux-gnueabihf" ]

build/config/linux/BUILD.gn

@@ -5,9 +5,19 @@
 import("//build/config/sysroot.gni")
 
 config("sdk") {
+  # Don't allow visible symbols from libc++ to be re-exported.
+  ldflags = [
+    "-nodefaultlibs",
+    "-lc++",
+    "-lc",
+    "-lm",
+    "-lclang_rt.builtins",
+    "-Wl,--exclude-libs=libc++.a",
+  ]
+
   if (sysroot != "") {
     cflags = [ "--sysroot=" + sysroot ]
-    ldflags = [ "--sysroot=" + sysroot ]
+    ldflags += [ "--sysroot=" + sysroot ]
 
     # Need to get some linker flags out of the sysroot.
     ldflags += [ exec_script("sysroot_ld_path.py",