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-ramoverflow.go

@@ -0,0 +1,9 @@
+package main
+
+var b [64 << 10]byte // 64kB
+
+func main() {
+	println("ptr:", &b[0])
+}
+
+// ERROR: program uses too much static RAM on this chip (RAM overflowed by {{[0-9]+}} bytes)