x86: fix get_cpu_info() when built with clang

x86: fix get_cpu_info() when built with clang

Clang understands the GCCism in use here, but still complains that sp is
unintialised.  In such cases, resort to the older version of this code, which
directly reads %rsp into the temporary variable.

Note that we still keep the GCCism in the default case, as it causes GCC to
create rather better assembly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>