Disable C++ exceptions in the Windows gn build

Disable C++ exceptions in the Windows gn build

This avoids unnecessary destructor cleanups in swiftshader code.  Posix
systems use -fno-exceptions, so this updates the MSVC gn args with the
equivalent, /EHs-c-.

I noticed this while investigating an unrelated issue.

Change-Id: Ic56e7560e7cb7617f8596b3ffda0444122425535
Reviewed-on: https://swiftshader-review.googlesource.com/14308
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Reid Kleckner <rnk@google.com>