Use dwarf exceptions on armv7 and aarch64

Dwarf is also the default in clang on i686, but libunwind currently
has an issue relating to unwinding dwarf exceptions on i686 on
windows (and less easily reproducible on other platforms),
see https://reviews.llvm.org/D38680.

This enables exceptions on aarch64, where they previously were
avoided/disabled.

When linking to a statically linked libunwind with support for
dwarf, one (currently) has to link in -lpsapi.

README.md

@@ -68,9 +68,6 @@ normal GCC/binutils based MinGW.
   the dllimport attributes.
 - The C++ libraries ([libcxxabi](http://libcxxabi.llvm.org/), [libcxx](http://libcxx.llvm.org/)) can only be linked statically
   at the moment.
-- The C++ libraries and unwinder aren't built with exceptions support for
-  ARM64 target at the moment. (The toolchain and libraries themselves do
-  support it though.)
 - The ARM64 target doesn't support thread local variables.
 
 Additionally, one may run into other minor differences between GCC and clang.