This shows up with with OpenMP offloading as libgomp since a couple
of months uses __atomic_compare_exchange (see PR for details),
causing link errors when the gcn libgomp.a is linked.
It also shows up with sollve_vv.

The implementation does a bit copy'n'paste from the current
implementation + calls the existing word/uint32_t-wide version
of the atomic intrinsic. The semantic is described at
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

In terms of the args, the _4 has:
DEF_SYNC_BUILTIN (BUILT_IN_ATOMIC_COMPARE_EXCHANGE_4,
                   "__atomic_compare_exchange_4",
                   BT_FN_BOOL_VPTR_PTR_I4_BOOL_INT_INT,
                   ATTR_NOTHROWCALL_LEAF_LIST)
and the arg names try to match the GCC manual.


OK for mainline?
Tested with libgomp + sollve_vv and -march=gfx908.

Tobias
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