Fix jl_get_specialization signature mismatch

[yuyichao]

Also fix a few other compiler warnings.

The signature mismatch was caught in a LTO build.

[yuyichao]

Since #18191 (more specifically b0e692a) is (marked for?) backporting, this commit needs to be too.

@tkelman the backport should remove the few changes in codegen.cpp that's changing integer types to suppress -Wsign-compare.

[tkelman]

@yuyichao do you mean just https://gist.github.com/c640cf74a21ab0a485be04a0c16d08f3 then?

[yuyichao]

Yes. With the declaration moved into julia_internal.h the patch should be correct as long as it compiles without warnings.