Update default mcpu values to not use SSSE3 and tune for pre-sandybridge CPUs

Update default mcpu values to not use SSSE3 and tune for pre-sandybridge CPUs

The "x86-64" value is the base 64-bit target which does not use SSE3+ as core2
did. The pentium4m mcpu value is the same as pentium4 but uses x86.td's
FeatureSlowBTMem attribute meaning "Bit testing of memory is slow" which is true
for pre-sandybridge CPUs (which are probably still prevalent, and in any case
matches the 64-bit code)
BUG=https://code.google.com/p/nativeclient/issues/detail?id=2205
R=jvoung@chromium.org

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=13362

[jvoung (off chromium), 2014-06-16 22:05:03]

lgtm

[Derek Schuff, 2014-06-17 15:59:23]

Committed patchset #1 manually as r13362 (presubmit successful).