mips: add GOMIPS=softfloat support

Previously, the compiler would default to hardfloat. This is not
supported by some MIPS CPUs.

This took me much longer than it should have because of a quirk in the
LLVM Mips backend: if the target-features string is not set (like during
LTO), the Mips backend picks the first function in the module and uses
that. Unfortunately, in the case of TinyGo this first function is
`llvm.dbg.value`, which is an LLVM intrinsic and doesn't have the
target-features string. I fixed it by adding a `-mllvm -mattr=` flag to
the linker.

builder/build.go

@@ -746,6 +746,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 				ldflags = append(ldflags, dependency.result)
 			}
 			ldflags = append(ldflags, "-mllvm", "-mcpu="+config.CPU())
+			ldflags = append(ldflags, "-mllvm", "-mattr="+config.Features()) // needed for MIPS softfloat
 			if config.GOOS() == "windows" {
 				// Options for the MinGW wrapper for the lld COFF linker.
 				ldflags = append(ldflags,

builder/builder_test.go

@@ -57,8 +57,10 @@ func TestClangAttributes(t *testing.T) {
 		{GOOS: "linux", GOARCH: "arm", GOARM: "6"},
 		{GOOS: "linux", GOARCH: "arm", GOARM: "7"},
 		{GOOS: "linux", GOARCH: "arm64"},
-		{GOOS: "linux", GOARCH: "mips"},
-		{GOOS: "linux", GOARCH: "mipsle"},
+		{GOOS: "linux", GOARCH: "mips", GOMIPS: "hardfloat"},
+		{GOOS: "linux", GOARCH: "mipsle", GOMIPS: "hardfloat"},
+		{GOOS: "linux", GOARCH: "mips", GOMIPS: "softfloat"},
+		{GOOS: "linux", GOARCH: "mipsle", GOMIPS: "softfloat"},
 		{GOOS: "darwin", GOARCH: "amd64"},
 		{GOOS: "darwin", GOARCH: "arm64"},
 		{GOOS: "windows", GOARCH: "amd64"},
@@ -69,6 +71,9 @@ func TestClangAttributes(t *testing.T) {
 		if options.GOARCH == "arm" {
 			name += ",GOARM=" + options.GOARM
 		}
+		if options.GOARCH == "mips" || options.GOARCH == "mipsle" {
+			name += ",GOMIPS=" + options.GOMIPS
+		}
 		t.Run(name, func(t *testing.T) {
 			testClangAttributes(t, options)
 		})

builder/library.go

@@ -169,6 +169,11 @@ func (l *Library) load(config *compileopts.Config, tmpdir string) (job *compileJ
 	case "mips":
 		args = append(args, "-fno-pic")
 	}
+	if config.Target.SoftFloat {
+		// Use softfloat instead of floating point instructions. This is
+		// supported on many architectures.
+		args = append(args, "-msoft-float")
+	}
 
 	var once sync.Once