tool: remove some gcc warnings in gccx

These two warnings seem to appear in gcc 4.6

-Wno-array-bounds: because, in generated code, local variables are stored in
a 1-length tail array.

-Wno-unused-but-set-variable: because unused parameters in Nit lead to
unused but initialized registers

Signed-off-by: Jean Privat <jean@pryen.org>