build: change reduce exports/static libstdc++ options for gitian and travis

build: change reduce exports/static libstdc++ options for gitian and travis

For Gitian releases:
  - Windows builds remain unchanged. libstdc++ was already linked statically.
  - OSX builds remain unchanged. libstdc++ is tied to the SDK and not worth
    messing with.
  - Linux builds now statically link libstdc++.

For Travis:
  - Match the previous behavior by adding --enable-reduce-exports as
  necessary.
  - Use static libstdc++ for the full Linux build.