Repair Linux-with-sysroot build after clang update

Clang switched to libc++ by default (apparently?) so force back to
libstdc++ and libgcc_s when building in the limited-minspec-sysroot
environment, as is done on Linux bots.

Carefully selected artisinal flags from
https://clang.llvm.org/docs/Toolchain.html.

Bug: crashpad:30
Change-Id: I2f4251853199e5b0c6950c288aac5e2947ad5c9c
Reviewed-on: https://chromium-review.googlesource.com/996452
Reviewed-by: Mark Mentovai <mark@chromium.org>

build/BUILD.gn

diff --git a/build/BUILD.gn b/build/BUILD.gn
index 0d75fc5..e9ff6ee 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -166,7 +166,12 @@
       # against a local copy of libc++ instead. As this build file only affects
       # the standalone Crashpad build, when this flag is set link libstdc++
       # statically to avoid the problem on the bots.
-      ldflags += [ "-static-libstdc++" ]
+      cflags += [ "-stdlib=libstdc++" ]
+      ldflags += [
+        "-rtlib=libgcc",
+        "-static-libstdc++",
+        "-stdlib=libstdc++",
+      ]
     }
   }