main: add -debug flag that controls debuginfo stripping

Debug information is useful in many case, but in the case of WebAssembly
it increases the binary size by a large amount. It also inhibits
compression of relocations. Therefore I've made debug information
optional and disabled by default.

(Debug information is still generated, but stripped from the binary at
the link step).

builder/build.go

@@ -100,7 +100,7 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		AutomaticStackSize: config.AutomaticStackSize(),
 		DefaultStackSize:   config.Target.DefaultStackSize,
 		NeedsStackObjects:  config.NeedsStackObjects(),
-		Debug:              config.Debug(),
+		Debug:              config.EmitDWARF(),
 		LLVMFeatures:       config.LLVMFeatures(),
 	}
 
@@ -537,6 +537,41 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 		ldflags = append(ldflags, lprogram.LDFlags...)
 	}
 
+	// Strip debug information if -debug is true. This is sometimes the default,
+	// such as with WebAssembly targets.
+	if !config.Debug() {
+		for _, tag := range config.BuildTags() {
+			if tag == "baremetal" {
+				// Don't use -debug=false on baremetal targets. It makes no
+				// sense: the debug information isn't flashed to the device
+				// anyway.
+				return fmt.Errorf("stripping debug information is unnecessary for baremetal targets")
+			}
+		}
+		if config.Target.Linker == "wasm-ld" {
+			// Don't just strip debug information, also compress relocations
+			// while we're at it. Relocations can only be compressed when debug
+			// information is stripped.
+			ldflags = append(ldflags, "--strip-debug", "--compress-relocations")
+		} else {
+			switch config.GOOS() {
+			case "linux":
+				// Either real linux or an embedded system (like AVR) that
+				// pretends to be Linux. It's a ELF linker wrapped by GCC in any
+				// case.
+				ldflags = append(ldflags, "-Wl,--strip-debug")
+			case "darwin":
+				// MacOS (darwin) doesn't have a linker flag to strip debug
+				// information. Apple expects you to use the strip command
+				// instead.
+				return errors.New("cannot remove debug information: MacOS doesn't suppor this linker flag")
+			default:
+				// Other OSes may have different flags.
+				return errors.New("cannot remove debug information: unknown OS: " + config.GOOS())
+			}
+		}
+	}
+
 	// Create a linker job, which links all object files together and does some
 	// extra stuff that can only be done after linking.
 	jobs = append(jobs, &compileJob{