Compile signal handler expressions in the loader thread

Compile signal handler expressions in the loader thread

Handle them similar to function declarations, except that we need to synthesize
the expression into a function declaration that includes the signal parameter
names. This is done quite similar to the code path in the new compiler.

Change-Id: I751081f7f1052692da6e2ed60c7f5c017372d829
Reviewed-by: Lars Knoll <lars.knoll@digia.com>