RFR (tedious) 8216022: Use #pragma once

[coleen.phillimore at oracle.com]

Summary: change include guards to #pragma once, except in generated 
header files.

Tested with mach5 for linux-x64{-debug}, solaris-sparc, macosx-x64, 
windows-x64, built aarch64 with cross compiler, and zero.

Ran tier1 and 2 tests.

The webrev is huge but there are only 3 lines changed in each header 
file.  So click on the patch.

I'll update the copyright headers with a script with the commit. Also, 
will do this after the shenandoah copyright headers are fixed.

Adrian: I included you to check your platforms.

Happy New Year!
Coleen