Add "sgn" to inbuilt-commands

sgn was already "prepared" in the blitz.c/blitz.h files but not connected to "bmx" via the compiler.

Output for const values is 100% compatible to vanilla BlitzMax:
- sgn(int) returns int value (1,0,-1)
- sgn(float) returns float value (1.0f, 0.0f, -1.0f)

Output for variables is not 100% compatibe to vanilla BlitzMax (current compiler approach seems not to enable this feature ... IMHO needs extension to do so)
- sgn(int) and sgn(float) return float values (1.0f, 0.0f, -1.0f) as per definition in "keywords" - Sgn#(v#)