RFR(8u): 8171974: Fix for R10 Register clobbering with usage of ExternalAddress

[Roman Kennke]

Can I please get a review for the following backport:
http://cr.openjdk.java.net/~rkennke/JDK-8171974-jdk8u/webrev.00/

This is a backport of:
https://bugs.openjdk.java.net/browse/JDK-8171974

The patch is identical with the original one:
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/8e139e328150

Except for the parts that are not yet present in 8u (all of
macroAssembler_x86_sha.cpp and some of stubGenerator_x86_64.cpp).

Testing: tier1 and tier2 without regressions

Is this ok to go into jdk8u?

Thanks,
Roman