RFR(XS) 8069030: support new PTRACE_GETREGSET

[Dean Long]

https://bugs.openjdk.java.net/browse/JDK-8069030

http://cr.openjdk.java.net/~dlong/8069030/webrev.00/

Testing on linux x86 by temporarily undefining PTRACE_GETREGS_REQ so 
that the new code
would get triggered, then stepping through "jstack -m <pid>" with gdb to 
make sure the new
code executed correctly.

dl