builder: hard code Clang compiler

At the moment, all targets use the Clang compiler to compile C and
assembly files. There is no good reason to make this configurable
anymore and in fact it will make future changes more complicated (and
thus more likely to have bugs). Therefore, I've removed support for
setting the compiler.

Note that the same is not true for the linker. While it makes sense to
standardize on the Clang compiler (because if Clang doesn't support a
target, TinyGo is unlikely to support it either), linkers will remain
configurable for the foreseeable future. One example is Xtensa, which is
supported by the Xtensa LLVM fork but doesn't have support in ld.lld
yet.

I've also fixed a bug in compileAndCacheCFile: it wasn't using the right
CFlags for caching purposes. This could lead to using stale caches. This
commit fixes that too.

builder/cc.go

@@ -17,7 +17,6 @@ import (
 	"strings"
 	"unicode"
 
-	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
 	"tinygo.org/x/go-llvm"
 )
@@ -57,7 +56,7 @@ import (
 //   depfile but without invalidating its name. For this reason, the depfile is
 //   written on each new compilation (even when it seems unnecessary). However, it
 //   could in rare cases lead to a stale file fetched from the cache.
-func compileAndCacheCFile(abspath, tmpdir string, cflags []string, config *compileopts.Config) (string, error) {
+func compileAndCacheCFile(abspath, tmpdir string, cflags []string, printCommands bool) (string, error) {
 	// Hash input file.
 	fileHash, err := hashFile(abspath)
 	if err != nil {
@@ -68,14 +67,12 @@ func compileAndCacheCFile(abspath, tmpdir string, cflags []string, config *compi
 	buf, err := json.Marshal(struct {
 		Path        string
 		Hash        string
-		Compiler    string
 		Flags       []string
 		LLVMVersion string
 	}{
 		Path:        abspath,
 		Hash:        fileHash,
-		Compiler:    config.Target.Compiler,
-		Flags:       config.CFlags(),
+		Flags:       cflags,
 		LLVMVersion: llvm.Version,
 	})
 	if err != nil {
@@ -131,10 +128,10 @@ func compileAndCacheCFile(abspath, tmpdir string, cflags []string, config *compi
 		// flags (for the assembler) is a compiler error.
 		flags = append(flags, "-Qunused-arguments")
 	}
-	if config.Options.PrintCommands {
-		fmt.Printf("%s %s\n", config.Target.Compiler, strings.Join(flags, " "))
+	if printCommands {
+		fmt.Printf("clang %s\n", strings.Join(flags, " "))
 	}
-	err = runCCompiler(config.Target.Compiler, flags...)
+	err = runCCompiler(flags...)
 	if err != nil {
 		return "", &commandError{"failed to build", abspath, err}
 	}