flashy_arm64 build

Summary: Compile arm64 version of flashy

Test Plan:
Checkout D61731216, build flashy ephemeral:

```
$ fbpkg build -E openbmc.utils.flashy --yes --configerator-path ~/configerator/
```

Ensure arm64 flashy binary is present
```
$ cd $(mktemp -d) && fbpkg fetch openbmc.utils.flashy:c84f8ad
...
$ file flashy_arm64
flashy_arm64: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, Go BuildID=1AdKZP4L9rfwwmxcE3C3/pY_LLowZsUX8QlcymBEz/yAXuELHk-8td4irRAKp1/M9d2EazNavpmMXz27q3x, stripped
```

Reviewed By: doranand

Differential Revision: D61730815

fbshipit-source-id: 7e5328f2f3604733f1577803444d105876e18afb

tools/flashy/build.sh

@@ -23,3 +23,4 @@ set -e
 cd "$(dirname "$0")"
 rm -rf build
 GOOS=linux GOARCH=arm GOARM=5 go build -ldflags="-w -s" -gcflags=all="-l" -o build/flashy
+GOOS=linux GOARCH=arm64 go build -ldflags="-w -s" -gcflags=all="-l" -o build/flashy_arm64