runtime on amd64, when it is compiled by clang. Some versions of clang
don't save and restore all callee registers, if a __builtin_eh_return()
intrinsic is used in a function. This is particularly bad on amd64.
Until the problem gets fixed by upstream, use an asm statement to force
clang to assume the registers in question are clobbered, when invoking
__builtin_eh_return(), so it will emit code to save and restore them.
This should fix the crashes reported on -current with some C++ programs,
particularly those that throw exceptions over multiple function
boundaries.
Reported by: stefanf
MFC after: 3 days
bfd6a1d202