picolibc: add include directory to build artefact

This is really just a preparatory commit for musl support. The idea is
to store not just the archive file (.a) but also an include directory.
This is optional for picolibc but required for musl, so the main purpose
of this commit is the refactor needed for this change.

compileopts/config.go

@@ -5,6 +5,7 @@ package compileopts
 import (
 	"errors"
 	"fmt"
+	"os"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -197,6 +198,29 @@ func (c *Config) RP2040BootPatch() bool {
 	return false
 }
 
+// LibcPath returns the path to the libc directory. The libc path will be either
+// a precompiled libc shipped with a TinyGo build, or a libc path in the cache
+// directory (which might not yet be built).
+func (c *Config) LibcPath(name string) (path string, precompiled bool) {
+	// Try to load a precompiled library.
+	precompiledDir := filepath.Join(goenv.Get("TINYGOROOT"), "pkg", c.Triple(), name)
+	if _, err := os.Stat(precompiledDir); err == nil {
+		// Found a precompiled library for this OS/architecture. Return the path
+		// directly.
+		return precompiledDir, true
+	}
+
+	// No precompiled library found. Determine the path name that will be used
+	// in the build cache.
+	var outname string
+	if c.CPU() != "" {
+		outname = name + "-" + c.Triple() + "-" + c.CPU()
+	} else {
+		outname = name + "-" + c.Triple()
+	}
+	return filepath.Join(goenv.Get("GOCACHE"), outname), false
+}
+
 // CFlags returns the flags to pass to the C compiler. This is necessary for CGo
 // preprocessing.
 func (c *Config) CFlags() []string {
@@ -208,12 +232,12 @@ func (c *Config) CFlags() []string {
 	case "picolibc":
 		root := goenv.Get("TINYGOROOT")
 		picolibcDir := filepath.Join(root, "lib", "picolibc", "newlib", "libc")
+		path, _ := c.LibcPath("picolibc")
 		cflags = append(cflags,
-			"-nostdlibinc",
+			"--sysroot="+path,
 			"-Xclang", "-internal-isystem", "-Xclang", filepath.Join(picolibcDir, "include"),
 			"-Xclang", "-internal-isystem", "-Xclang", filepath.Join(picolibcDir, "tinystdio"),
 		)
-		cflags = append(cflags, "-I"+filepath.Join(root, "lib/picolibc-include"))
 	case "wasi-libc":
 		root := goenv.Get("TINYGOROOT")
 		cflags = append(cflags, "--sysroot="+root+"/lib/wasi-libc/sysroot")