thumbv7m target: make linker flavor match x86

This probably isn't significant, but might as well match between
targets.

Author: tylerwhall

rust/thumbv7m-zephyr-eabi.json

@@ -10,10 +10,10 @@
   "arch": "arm",
   "data-layout": "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64",
   "emit-debug-gdb-scripts": false,
-  "env": "",
+  "env": "gnu",
   "executables": true,
-  "linker": "rust-lld",
-  "linker-flavor": "ld.lld",
+  "linker-flavor": "gcc",
+  "linker-is-gnu": true,
   "llvm-target": "thumbv7m-none-gnueabi",
   "max-atomic-width": 32,
   "os": "zephyr",