Do not add -fno-exceptions without -funwind-tables

Adding just -fno-exceptions breaks libunwind in quite mysterious way
when it's there, but exception handling doesn't work because of dummy
unwind tables.

Also as using exceptions implies references to symbols defined in
libcxx, abort build of libcxxabi as shared library if we have to keep
exceptions (when compiler supports -fno-exceptions, but not
-funwind-tables; one example would be a cross-compiler, in which case
testing for -funwind-tables flag by CMake actually requires libunwind to
be available before it's built).

llvm-svn: 229427