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.

goenv/goenv.go

@@ -30,8 +30,11 @@ var Keys = []string{
 }
 
 func init() {
-	if Get("GOARCH") == "arm" {
+	switch Get("GOARCH") {
+	case "arm":
 		Keys = append(Keys, "GOARM")
+	case "mips", "mipsle":
+		Keys = append(Keys, "GOMIPS")
 	}
 }
 
@@ -128,6 +131,13 @@ func Get(name string) string {
 		// difference between ARMv6 and ARMv7. ARMv6 binaries are much smaller,
 		// especially when floating point instructions are involved.
 		return "6"
+	case "GOMIPS":
+		gomips := os.Getenv("GOMIPS")
+		if gomips == "" {
+			// Default to hardfloat (this matches the Go toolchain).
+			gomips = "hardfloat"
+		}
+		return gomips
 	case "GOROOT":
 		readGoEnvVars()
 		return goEnvVars.GOROOT