main: implement tinygo lldb subcommand

LLDB mostly works on most platforms, but it is still lacking in some
features. For example, it doesn't seem to support RISC-V yet (coming in
LLVM 12), it only partially supports AVR (no stacktraces), and it
doesn't seem to support the Ctrl-C keyboard command when running a
binary for another platform (e.g. with GOOS=arm64). However, it does
mostly work, even on baremetal systems.

builder/tools.go

@@ -24,7 +24,7 @@ func runCCompiler(flags ...string) error {
 	}
 
 	// Compile this with an external invocation of the Clang compiler.
-	return execCommand(commands["clang"], flags...)
+	return execCommand("clang", flags...)
 }
 
 // link invokes a linker with the given name and flags.
@@ -38,8 +38,8 @@ func link(linker string, flags ...string) error {
 	}
 
 	// Fall back to external command.
-	if cmdNames, ok := commands[linker]; ok {
-		return execCommand(cmdNames, flags...)
+	if _, ok := commands[linker]; ok {
+		return execCommand(linker, flags...)
 	}
 
 	cmd := exec.Command(linker, flags...)