RFR: 8183574: Unify the is_power_of_2 functions

[Stefan Karlsson]

Hi all,

Please review this patch to consolidate all is_power_of_2 functions into 
one template function and place it in the existing powerOfTwo.hpp file.

To resolve a circular dependency between globalDefinitions.hpp and 
powerOfTwo.hpp, I also moved exact_log2 and exact_log2_long. These 
functions take a power-of-two value and returns the log2 value, so I 
think it makes sense to move them to powerOfTwo.hpp.

https://cr.openjdk.java.net/~stefank/8183574/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8183574

Tested with tier1,2,3
Also built locally with 
{fastdebug,release}x{minimal,zero,aarch64,ppc,s390,arm32,shenandoah}

Thanks,
StefanK