adapt the thumb target specs to upstream linker-flavor changes

.cargo/config

@@ -1,11 +0,0 @@
-[target.thumbv6m-none-eabi]
-rustflags = ["-C", "link-arg=-nostartfiles"]
-
-[target.thumbv7m-none-eabi]
-rustflags = ["-C", "link-arg=-nostartfiles"]
-
-[target.thumbv7em-none-eabi]
-rustflags = ["-C", "link-arg=-nostartfiles"]
-
-[target.thumbv7em-none-eabihf]
-rustflags = ["-C", "link-arg=-nostartfiles"]

thumbv6m-linux-eabi.json

@@ -12,11 +12,14 @@
     "executables": true,
     "features": "+strict-align",
     "linker": "arm-none-eabi-gcc",
+    "linker-flavor": "gcc",
     "llvm-target": "thumbv6m-none-eabi",
     "max-atomic-width": 0,
     "os": "linux",
     "panic-strategy": "abort",
-    "pre-link-args": ["-nostartfiles"],
+    "pre-link-args": {
+        "gcc": ["-nostartfiles"]
+    },
     "relocation-model": "static",
     "target-endian": "little",
     "target-pointer-width": "32",

thumbv7em-linux-eabi.json

@@ -11,11 +11,14 @@
     "env": "",
     "executables": true,
     "linker": "arm-none-eabi-gcc",
+    "linker-flavor": "gcc",
     "llvm-target": "thumbv7em-none-eabi",
     "max-atomic-width": 32,
     "os": "linux",
     "panic-strategy": "abort",
-    "pre-link-args": ["-nostartfiles"],
+    "pre-link-args": {
+        "gcc": ["-nostartfiles"]
+    },
     "relocation-model": "static",
     "target-endian": "little",
     "target-pointer-width": "32",

thumbv7em-linux-eabihf.json

@@ -12,11 +12,14 @@
     "executables": true,
     "features": "+vfp4,+d16,+fp-only-sp",
     "linker": "arm-none-eabi-gcc",
+    "linker-flavor": "gcc",
     "llvm-target": "thumbv7em-none-eabihf",
     "max-atomic-width": 32,
     "os": "linux",
     "panic-strategy": "abort",
-    "pre-link-args": ["-nostartfiles"],
+    "pre-link-args": {
+        "gcc": ["-nostartfiles"]
+    },
     "relocation-model": "static",
     "target-endian": "little",
     "target-pointer-width": "32",

thumbv7m-linux-eabi.json

@@ -11,11 +11,14 @@
     "env": "",
     "executables": true,
     "linker": "arm-none-eabi-gcc",
+    "linker-flavor": "gcc",
     "llvm-target": "thumbv7m-none-eabi",
     "max-atomic-width": 32,
     "os": "linux",
     "panic-strategy": "abort",
-    "pre-link-args": ["-nostartfiles"],
+    "pre-link-args": {
+        "gcc": ["-nostartfiles"]
+    },
     "relocation-model": "static",
     "target-endian": "little",
     "target-pointer-width": "32",