Raise x86 arch requirement to prescott.

Atomic functions used in external/libcxx/include/atomic when compiled with Clang
will require intrinsic functions exist only for prescott or newer CPUs.

BUG: 17530542
Change-Id: I0c9660ed2ffa75b940981eb8165d88934b39aec5
Merge "Raise x86 arch requirement to prescott."