Use gcc / clang overflow builtins. (#142)

* Use gcc / clang overflow builtins.

This avoids expensive checks for overflow that employ undefined
behavior.

This is a step along the way towards replacing the old hand-written
assembler that did the same thing in terms of using the CPU's
overflow detection.

* Remove unimplemented SPARC asm for multiplication, divide, and remainder.

This wasn't implemented before, and for multiplication, it is now
implemented for gcc and friends using overflow detection.

* Remove USE_INLINE_ARITH.

Now that we have the compiler built-ins for detecting overflow,
we don't need custom assembly for it for each platform.

For now, we keep, but still don't use, the code that do a hot
path through the dispatch loop for some math. This code isn't
actually running or in use, but it is separate from how the
other inline arithmetic was being performed. These are the
`fast_op_*` functions that are implemented in assembler.

src/dspSPARC.il

@@ -131,80 +131,6 @@ done:				!		else done;
 	nop
 .end
 
-/*
-	***************************************************************
-	Arithmetic Opcode Helpers
-	***************************************************************
-*/
-
-.inline	_sub32,8
-	subcc	%o0,%o1,%o0	! result = arg0 - arg1
-	bvs	diff_err
-	nop
-.end
-
-.inline	_isub32,8
-	subcc	%o0,%o1,%o0	! result = arg0 - arg1
-	bvs	idiff_err
-	nop
-.end
-
-.inline	_sub32n,8
-	subcc	%o0,%o1,%o0	! result = arg0 - arg1
-	bvs	diff_err2
-	nop
-.end
-
-.inline	_mpy32,8
-	ba	mpy_err
-	nop
-.end
-
-.inline	_impy32,8
-	ba	impy_err
-	nop
-.end
-
-.inline	_quot32,8
-	ba	quot_err
-	nop
-.end
-
-.inline	_iquot32,8
-	ba	iquot_err
-	nop
-.end
-
-.inline	_rem32,8
-	ba	rem_err
-	nop
-.end
-
-.inline	_irem32,8
-	ba	irem_err
-	nop
-.end
-
-
-.inline	_plus32,8
-	addcc	%o0,%o1,%o0	! result = arg0 + arg1
-	bvs	plus_err
-	nop
-.end
-
-.inline	_iplus32,8
-	addcc	%o0,%o1,%o0	! result = arg0 + arg1
-	bvs	iplus_err
-	nop
-.end
-
-.inline	_iplus32n,8
-	addcc	%o0,%o1,%o0	! result = arg0 + arg1
-	bvs	iplusn_err
-	nop
-.end
-
-
 /*
 	***************************************************************
 	Inline Assembly help for dispatcher.