aarch64: RFR: Block zeroing by 'DC ZVA'

[Long Chen]

Hi

Please review this patch making use of DC ZVA to do block zeroing.

http://people.linaro.org/~long.chen/block_zeroing/block_zeroing.patch

I’m sorry that I can’t produce a test case matching the ‘clear_array’
pattern showing obvious improvement. However, generating ‘DC ZVA’ should be
the right thing to do as it usually has better cache behaviors. Besides,
gcc and linux’s memset have been using ‘DC ZVA’.

The ArrayFillByte case benefits from ‘DC ZVA’ when the array length is
large.
Test, http://people.linaro.org/~long.chen/block_zeroing/ArrayFillByte.java
Performance result,
http://people.linaro.org/~long.chen/block_zeroing/BlockZeroing.html

Tested with jtreg hotspot and langtools.

Thanks!
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