Rename unwind.h to unwind-gcc.h

[Douglas B Rupp]

VxWorks7 has introduced an incompatible unwind.h header file that 
conflicts with gcc's unwind.h (which is a build time header file linked 
to the appropriate source header).

I'd like to propose renaming gcc's unwind.h to unwind-gcc.h

There are 54 occurrences in the gcc source tree, including the testsuite 
and one in the gdb testsuite.

I have a patch ready to submit, but wished to get a preliminary reaction 
and have a discussion on issues I might have failed to consider.

--Douglas Rupp