RFR(xs): 8145114: const-correctness for ucontext_t* reading functions

[Thomas Stüfe]

Hi all,

please review this large-but-trivial change:

Bug: https://bugs.openjdk.java.net/browse/JDK-8145114
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8145114-const-correctness-ucontext/webrev.00/webrev/index.html

Even though many files are involved, the changes are very trivial. The
change takes care of declaring ucontext_t pointers as const for all utility
functions which do not change the context (which are most of them).

This fix was motivated by me wasting time in an error situation on AIX
where I accidentally changed the register values in the context before
printing them to the hs-err file.

And, like pulling a thread from a ball of yarn, the change got bigger and
bigger... like it is once one starts to fix const issues. But the fixes in
this case involve the bottom functions and should not force any new casts
upon anyone.

I did build the change on the following platforms:
AIX
Linux x64 (with and without ZERO)
Linux ppc
OSX
Solaris Sparc and x64

and all did build successfully.

Kind Regards, Thomas