llvm: Set use-soft-float and noimplicitfloat on functions for soft float.

Closes #10961.

src/codegen/llvm.zig

@@ -3108,6 +3108,30 @@ pub const Object = struct {
                 .value = .empty,
             } }, &o.builder);
         }
+        if (target.floatAbi() == .soft) {
+            // `use-soft-float` means "use software routines for floating point computations". In
+            // other words, it configures how LLVM lowers basic float instructions like `fcmp`,
+            // `fadd`, etc. The float calling convention is configured on `TargetMachine` and is
+            // mostly an orthogonal concept, although obviously we do need hardware float operations
+            // to actually be able to pass float values in float registers.
+            //
+            // Ideally, we would support something akin to the `-mfloat-abi=softfp` option that GCC
+            // and Clang support for Arm32 and CSKY. We don't currently expose such an option in
+            // Zig, and using CPU features as the source of truth for this makes for a miserable
+            // user experience since people expect e.g. `arm-linux-gnueabi` to mean full soft float
+            // unless the compiler has explicitly been told otherwise. (And note that our baseline
+            // CPU models almost all include FPU features!)
+            //
+            // Revisit this at some point.
+            try attributes.addFnAttr(.{ .string = .{
+                .kind = try o.builder.string("use-soft-float"),
+                .value = try o.builder.string("true"),
+            } }, &o.builder);
+
+            // This prevents LLVM from using FPU/SIMD code for things like `memcpy`. As for the
+            // above, this should be revisited if `softfp` support is added.
+            try attributes.addFnAttr(.noimplicitfloat, &o.builder);
+        }
     }
 
     fn resolveGlobalUav(