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.

.circleci/config.yml

@@ -24,7 +24,6 @@ commands:
                 libclang<<parameters.llvm>>-dev \
                 lld<<parameters.llvm>> \
                 gcc-arm-linux-gnueabihf \
-                binutils-arm-none-eabi \
                 libc6-dev-armel-cross \
                 gcc-aarch64-linux-gnu \
                 libc6-dev-arm64-cross \

.travis.yml

@@ -11,11 +11,9 @@ matrix:
 addons:
   homebrew:
     update: true
-    taps: ArmMbed/homebrew-formulae
     packages:
     - llvm@8
     - qemu
-    - arm-none-eabi-gcc
 
 install:
   - curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh

Dockerfile

@@ -59,7 +59,7 @@ COPY --from=tinygo-base /go/src/github.com/tinygo-org/tinygo/lib /go/src/github.
 
 RUN cd /go/src/github.com/tinygo-org/tinygo/ && \
     apt-get update && \
-    apt-get install -y apt-utils python3 make binutils-arm-none-eabi clang-8 && \
+    apt-get install -y apt-utils python3 make clang-8 && \
     make gen-device-nrf && make gen-device-stm32 && \
     apt-get remove -y python3 make && \
     apt-get autoremove -y && \
@@ -74,7 +74,7 @@ COPY --from=tinygo-base /go/src/github.com/tinygo-org/tinygo/lib /go/src/github.
 
 RUN cd /go/src/github.com/tinygo-org/tinygo/ && \
     apt-get update && \
-    apt-get install -y apt-utils python3 make binutils-arm-none-eabi clang-8 binutils-avr gcc-avr avr-libc && \
+    apt-get install -y apt-utils python3 make clang-8 binutils-avr gcc-avr avr-libc && \
     make gen-device && \
     apt-get remove -y python3 make && \
     apt-get autoremove -y && \

commands_macos.go

@@ -6,6 +6,6 @@ package main
 var commands = map[string]string{
 	"ar":      "llvm-ar",
 	"clang":   "clang-8",
-	"ld.lld":  "ld.lld-8",
+	"ld.lld":  "ld.lld",
 	"wasm-ld": "wasm-ld",
 }

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",