The `__cpp_exception` symbol is now defined in libunwind instead of
compiler-rt. This is moved for a few reasons, but the primary reason is
that compiler-rt is linked duplicate-ly into all shared objects meaning
that it's not suitable for define-once symbols such as
`__cpp_exception`. By moving the definition to the user of the symbol,
libunwind itself, that guarantees that the symbol should be defined
exactly once and only when appropriate. A secondary reason for this
movement is that it avoids the need to compile compiler-rt twice: once
with exception and once without, and instead the same build can be used
for both exceptions-and-not.
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