Use --gcc-toolchain instead of -B to find libgcc

Caused by https://reviews.llvm.org/D97993.

Bug: 1191244
Change-Id: Ieb2b8487d6fddf9e0c310f68ed6680c39088f3b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2781154
Auto-Submit: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#865546}

build/config/android/BUILD.gn

@@ -99,7 +99,8 @@ config("compiler") {
 # :runtime_library vs. :compiler.
 config("runtime_library") {
   # Let the linker find libgcc.a.
-  ldflags = [ "-B" + rebase_path(android_toolchain_root, root_build_dir) ]
+  ldflags = [ "--gcc-toolchain=" +
+              rebase_path(android_toolchain_root, root_build_dir) ]
 
   libs = []