x86-64: Use RDX_LP on __x86_shared_non_temporal_threshold [BZ #25966]

Since __x86_shared_non_temporal_threshold is defined as

long int __x86_shared_non_temporal_threshold;

and long int is 4 bytes for x32, use RDX_LP to compare against
__x86_shared_non_temporal_threshold in assembly code.

(cherry picked from commit 55c7bcc7)