RFR: 8270830: Aarch64: Use stp to initialize object on C1

[Wang Huang]

Here is a trivail patch on aarch64. 
It is found that in C1_MacroAssembler::initialize_object which we can use` stp `to replace` str`.
```c++
@@ -250,13 +249,16 @@ void C1_MacroAssembler::initialize_object(Register obj, Register klass, Register
        for (int i = -unroll; i < 0; i++) {
          if (-i == remainder)
            bind(entry_point);
- str(zr, Address(rscratch1, i * wordSize));
+ stp(zr, zr, Address(rscratch1, i * 2 * BytesPerWord));

-------------

Commit messages:
 - 8270830: Aarch64: Use stp to initialize object on C1

Changes: https://git.openjdk.java.net/jdk/pull/4808/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4808&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270830
  Stats: 12 lines in 1 file changed: 4 ins; 2 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4808.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4808/head:pull/4808

PR: https://git.openjdk.java.net/jdk/pull/4808