Change unreachable code to error. Remove unreachable where the compiler now can detect reachability.

lib/std/io/os/getcwd.c3

@@ -35,7 +35,7 @@ macro String! getcwd(Allocator* using = mem::heap())
 			return res.copy(using);
 
 		$default:
-			unreachable("'getcwd' not available");
+		    return IoError.UNSUPPORTED_OPERATION?;
 	$endswitch
 }
 

lib/std/math/math_nolibc/cos.c3

@@ -37,7 +37,6 @@ fn float _cosf(float x) @extern("cosf") @weak @nostrip
 		case 2: return  -__cosdf(y);
 		default: return __sindf(y);
 	}
-	unreachable();
 }
 
 /* origin: FreeBSD /usr/src/lib/msun/src/s_cos.c */

lib/std/math/math_nolibc/sin.c3

@@ -70,7 +70,6 @@ fn float _sinf(float x) @weak @extern("sinf") @nostrip
 		case 2: return  __sindf(-y);
 		default: return -__cosdf(y);
 	}
-	unreachable();
 }
 
 /* origin: FreeBSD /usr/src/lib/msun/src/s_sin.c */