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.

testdata/errors/linker-undefined.go

@@ -0,0 +1,11 @@
+package main
+
+func foo()
+
+func main() {
+	foo()
+	foo()
+}
+
+// ERROR: linker-undefined.go:6: linker could not find symbol {{_?}}main.foo
+// ERROR: linker-undefined.go:7: linker could not find symbol {{_?}}main.foo