cbe: legalize safety instructions in non-zig1 builds

This is valid if the bootstrap dev env doesn't need to support runtime
safety.  Another solution can always be implemented if needs change.

src/arch/riscv64/CodeGen.zig

@@ -51,7 +51,7 @@ const Instruction = encoding.Instruction;
 
 const InnerError = CodeGenError || error{OutOfRegisters};
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,

src/arch/wasm/CodeGen.zig

@@ -31,7 +31,7 @@ const libcFloatSuffix = target_util.libcFloatSuffix;
 const compilerRtFloatAbbrev = target_util.compilerRtFloatAbbrev;
 const compilerRtIntAbbrev = target_util.compilerRtIntAbbrev;
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,

src/codegen/c.zig

@@ -4,6 +4,7 @@ const assert = std.debug.assert;
 const mem = std.mem;
 const log = std.log.scoped(.c);
 
+const dev = @import("../dev.zig");
 const link = @import("../link.zig");
 const Zcu = @import("../Zcu.zig");
 const Module = @import("../Package/Module.zig");
@@ -21,7 +22,12 @@ const BigIntLimb = std.math.big.Limb;
 const BigInt = std.math.big.int;
 
 pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
-    return null;
+    return if (dev.env.supports(.legalize)) comptime &.initMany(&.{
+        .expand_intcast_safe,
+        .expand_add_safe,
+        .expand_sub_safe,
+        .expand_mul_safe,
+    }) else null; // we don't currently ask zig1 to use safe optimization modes
 }
 
 pub const CType = @import("c/Type.zig");

src/codegen/spirv.zig

@@ -28,7 +28,7 @@ const SpvAssembler = @import("spirv/Assembler.zig");
 
 const InstMap = std.AutoHashMapUnmanaged(Air.Inst.Index, IdRef);
 
-pub fn legalizeFeatures(_: *const std.Target) ?*const Air.Legalize.Features {
+pub fn legalizeFeatures(_: *const std.Target) *const Air.Legalize.Features {
     return comptime &.initMany(&.{
         .expand_intcast_safe,
         .expand_add_safe,

src/dev.zig

@@ -1,5 +1,8 @@
 pub const Env = enum {
     /// zig1 features
+    /// - `-ofmt=c` only
+    /// - `-OReleaseFast` or `-OReleaseSmall` only
+    /// - no `@setRuntimeSafety(true)`
     bootstrap,
 
     /// zig2 features