libc++ uses a predefined macro to control whether to use exceptions, so
defining this macro is unnecessary. Defining _HAS_EXCEPTIONS to 0 also
breaks libc++ because it depends on MSVC headers that only provide
certain declarations if _HAS_EXCEPTIONS is 1.

Bug: chromium:801780
Change-Id: I0d0c04e55e1e91c3ce7fe90d15b55de0cd7a21ac
Reviewed-on: https://swiftshader-review.googlesource.com/16688Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Peter Collingbourne <pcc@google.com>