Repair Linux-with-sysroot build after clang update
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 <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/mini_chromium,chromium/mini_chromium,chromium/mini_chromium