arm: use the lld linker

LLD version 8 has added support for armv6m:
    https://reviews.llvm.org/D55555
This means we can use LLD instead of arm-none-eabi-ld, eliminating our
dependency on GNU binutils.

There are small differences in code size, but never more than a few
bytes.

targets/arm.ld

@@ -54,6 +54,7 @@ SECTIONS
 
     /DISCARD/ :
     {
+        *(.ARM.exidx)      /* causes 'no memory region specified' error in lld */
         *(.ARM.exidx.*)    /* causes spurious 'undefined reference' errors */
     }
 }

targets/cortex-m.json

@@ -4,7 +4,7 @@
 	"goarch": "arm",
 	"compiler": "clang-8",
 	"gc": "marksweep",
-	"linker": "arm-none-eabi-ld",
+	"linker": "ld.lld",
 	"rtlib": "compiler-rt",
 	"cflags": [
 		"-Oz",