RFR: aarch64: improve _unsafe_arraycopy stub routine

[Felix Yang]

Hi,

    Please review webrev:
http://cr.openjdk.java.net/~fyang/8152840/webrev.00/
    JIRA issue: https://bugs.openjdk.java.net/browse/JDK-8152840

    Patch improves _unsafe_arraycopy stub by checking the alignment of the
src, dest, and count, and call some faster arraycopy stubs (say
StubRoutines::_jshort_arraycopy).  About 1%+ performance gain on Spark
Terasort benchmark by this improvement.

    Here is the disassembly of the new stub routine code:
  StubRoutines::unsafe_arraycopy [0x0000007f8c93a000, 0x0000007f8c93a040[
(64 bytes)
   0x0000007f8c93a000: stp       x29, x30, [sp,#-16]!
   0x0000007f8c93a004: mov       x29, sp
   0x0000007f8c93a008: orr       x8, x0, x1
   0x0000007f8c93a00c: orr       x8, x8, x2
   0x0000007f8c93a010: and       x8, x8, #0x7
   0x0000007f8c93a014: cbz       x8, Stub::unsafe_arraycopy+56
0x0000007f8c93a038
   0x0000007f8c93a018: and       x8, x8, #0x3
   0x0000007f8c93a01c: cbz       x8, Stub::unsafe_arraycopy+48
0x0000007f8c93a030
   0x0000007f8c93a020: tbz       w8, #0, Stub::unsafe_arraycopy+40
0x0000007f8c93a028
   0x0000007f8c93a024: b Stub::jbyte_arraycopy+8 0x0000007f8c938d68
   0x0000007f8c93a028: lsr       x2, x2, #1
   0x0000007f8c93a02c: b Stub::jshort_arraycopy+8 0x0000007f8c938fd4
   0x0000007f8c93a030: lsr       x2, x2, #2
   0x0000007f8c93a034: b Stub::jint_arraycopy+8 0x0000007f8c9392fc
   0x0000007f8c93a038: lsr       x2, x2, #3
   0x0000007f8c93a03c: b Stub::arrayof_jlong_arraycopy+8 0x0000007f8c9393d4

    Tested with JTreg hotspot & langtools, is it OK?
    BTW: The _unsafe_arraycopy and _generic_arraycopy stubs are missing for
jdk8u, I would like to backport them from jdk9 later.

Thanks,
Felix
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