Makefile does not detect TARGET_ARCH correctly on FreeBSD

On FreeBSD 12.2 amd64 the Makefile does not correctly detect TARGET_ARCH correctly (`uname -m` returns `amd64` not `x86_64`) which results in the Makefile failing with the error: `Makefile:37: *** This system's architecture amd64 isn't supported. Stop.`

It is fairly easy to get round this by setting GOARCH manually (eg. `GOARCH=amd64 gmake`) however the following minor patch fixes this.

```patch
diff --git a/Makefile b/Makefile
index 2d545cd..48eaf1b 100644
--- a/Makefile
+++ b/Makefile
@@ -25,6 +25,8 @@ ifneq ($(GOARCH),)
TARGET_ARCH ?= $(GOARCH)
else ifeq ($(LOCAL_ARCH),x86_64)
TARGET_ARCH ?= amd64
+else ifeq ($(LOCAL_ARCH),amd64)
+ TARGET_ARCH ?= amd64
else ifeq ($(LOCAL_ARCH),i686)
TARGET_ARCH ?= amd64
else ifeq ($(shell echo $(LOCAL_ARCH) | head -c 5),armv8)
```