linker: Require an exact matching signature when looking for prototypes.

linker: Require an exact matching signature when looking for prototypes.

Fixes piglit test glsl-override-builtin.  The linker incorrectly found
the prototype for the float signature, rather than adding a new
prototype with the int return type.  This caused ir_calls with type int
to have their callees set to the float signature, triggering an assert.