Truncate rather than round to nearest when performing float-to-integer

Truncate rather than round to nearest when performing float-to-integer
conversions for external array types, which implement the Typed Array
spec. (Revision of http://codereview.chromium.org/6315004 .)

Prefer SSE2 code path on x86 processors. Non-SSE2 processors now make
a slow runtime call for float-to-int conversions. Use SSE3 for 32-bit
signed and unsigned int array types where possible.

The movement of code from ic-arm.cc to stub-cache-arm.cc caused the
VFP3 code path to be tested for the first time. Fixed bugs in the
register usage and in the constant value stored into integer arrays
for NaN and +/-Infinity.

Added new truncation test to test-api.cc. Storage of NaN and +/-Inf
was already covered. Ran unit tests on x86, x64 and ARM simulator.
Tested ia32 and x64 code in Chromium on Mac and Linux respectively
with Typed Array unit tests and WebGL content.

BUG=http://code.google.com/p/chromium/issues/detail?id=50972
TEST=test-api/ExternalArrays

Review URL: http://codereview.chromium.org/6303012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6431 ce2b1a6d-e550-0410-aec6-3dcde31c8c00