BaseTools/tools_def CLANG3x: ignore unknown warning options

BaseTools/tools_def CLANG3x: ignore unknown warning options

Ironically, disabling warnings in the OpensslLib library build is
causing breakage when using the CLANG35 toolchain to build for ARM:

error: unknown warning option '-Werror=maybe-uninitialized'; did you mean '-Werror=uninitialized'? [-Werror,-Wunknown-warning-option]

So let's add -Wno-unknown-warning-option to the list of warnings to
ignore when using Clang 3.5, and move the same option from the x86
specific list to the shared list for Clang 3.8.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>