BUG/MEDIUM: threads: fix the double CAS implementation for ARMv7

Commit f61f0cb ("MINOR: threads: Introduce double-width CAS on x86_64
and arm.") introduced the double CAS. But the ARMv7 version is bogus,
it uses the value of the pointers instead of dereferencing them. When
lucky, it simply doesn't build due to impossible registers combinations.
Otherwise it will immediately crash at run time when facing traffic.

No backport is needed, this bug was introduced in 1.9-dev.
1 file changed