More C90 compatibility fixes

This silences the following gcc/clang warnings from MonoJenkins:
sgen-tarjan-bridge.c:395:2: warning: ISO C90 forbids mixed declarations and code
proflog.c:1942:2: warning: ISO C90 forbids mixed declarations and code
proflog.c:1999:3: warning: ISO C90 forbids mixed declarations and code
decode.c:2409:6: warning: ISO C90 forbids mixed declarations and code
decode.c:2433:6: warning: ISO C90 forbids mixed declarations and code

Moving the declarations to the beginning of the blocks is sufficient
to silence the warnings.

This change is released under the MIT license.
Fix non-prototype function declarations

MonoJenkins repeatedly complains about:
mono-counters.c:xxx:1: warning: function declaration isn't a prototype
[-Wstrict-prototypes]

This is caused by the missing declaration of the function arguments.

This change is released under the MIT license.
Merge pull request #1059 from ranma42/jenkins-warnings-cleanup

Cleanup of some MonoJenkins warnings