[15] RFR 8250588: Shenandoah: LRB needs to save/restore fp registers for runtime call

[Zhengyu Gu]

We recently experienced some test failures with 
VarHandleTestMethodHandleAccessDouble.java and 
VarHandleTestMethodHandleAccessfloat.java, where appeared to have 
corrupted fp registers.

It turns out that we have to save fp registers in LRB when making 
runtime calls.


Bug: https://bugs.openjdk.java.net/browse/JDK-8250588
Weberev: http://cr.openjdk.java.net/~zgu/JDK-8250588/webrev.00/

Test:
   hotspot_gc_shenandoah
   tier1 with Shenandoah


Thanks,

-Zhengyu