RFR (S): 8152856: Xcode 7.3 -Wshift-negative-value compile failure on Mac OS X

[Gerard Ziemski]

hi all,

Please review this small fix that enables Xcde 7.3 support for building hotspot, which involved 2 things:

- modify gcc.make to add support for 7.3 version of clang compiler
- modify src/cpu/x86/vm/stubGenerator_x86_64.cpp, src/share/vm/oops/klass.hpp and src/share/vm/opto/library_call.cpp to cast 4 instances of signed int to unsigned when using shift operator to avoid "-Wshift-negative-value" warning.

https://bugs.openjdk.java.net/browse/JDK-8152856
http://cr.openjdk.java.net/~gziemski/8152856_rev1

Passes JPRT hotspot on all platforms