builder: interpret linker error messages

This shows nicely formatted error messages for missing symbol names and
for out-of-flash, out-of-RAM conditions (on microcontrollers with
limited flash/RAM).

Unfortunately the missing symbol name errors aren't available on Windows
and WebAssembly because the linker doesn't report source locations yet.
This is something that I could perhaps improve in LLD.

builder/build.go

@@ -779,7 +779,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 			}
 			err = link(config.Target.Linker, ldflags...)
 			if err != nil {
-				return &commandError{"failed to link", result.Executable, err}
+				return err
 			}
 
 			var calculatedStacks []string