RFR(S): 8250635: MethodArityHistogram should use Compile_lock in favour of fancy checks

[Schmidt, Lutz]

Dear all, 

may I please request reviews for this small enhancement? Instead of calling a method doing complicated and fancy (hard to understand) checks, the iteration over all nmethods is now protected by holding the Compile_lock in addition to the CodeCache_lock.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8250635
Webrev: https://cr.openjdk.java.net/~lucy/webrevs/8250635.00/

Thank you!
Lutz