targets: Fix max-atomic-width target setting

Nightly rustc throws compiler errors since upstream commit
3be40b22c834 ("Fix AtomicPtr::from_mut align check: Avoid
generic arg in const expr.") [1].

They seem to be related to some discrepancies in the alignment
requirements for pointer types on AVR.  While this should
probably be investigated at some point, for now let's set
`max-atomic-width` to 8 to make everything build again but
still retain at least some atomic types (those aren't actual
atomics but intrinsics that are lowered to critical sections
for atomic access).

[1]: rust-lang/rust@3be40b2
Signed-off-by: Rahix <rahix@rahix.de>

chips/atmega1280-hal/avr-atmega1280.json

@@ -9,6 +9,8 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
+
   "executables": true,
   "linker": "avr-gcc",
   "linker-flavor": "gcc",

chips/atmega168-hal/avr-atmega168.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,
 

chips/atmega2560-hal/avr-atmega2560.json

@@ -9,6 +9,8 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
+
   "executables": true,
   "linker": "avr-gcc",
   "linker-flavor": "gcc",

chips/atmega328p-hal/avr-atmega328p.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,
 

chips/atmega32u4-hal/avr-atmega32u4.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,
 

chips/atmega48p-hal/avr-atmega48p.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,
 

chips/attiny85-hal/avr-attiny85.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,
 

chips/attiny88-hal/avr-attiny88.json

@@ -9,6 +9,7 @@
   "target-vendor": "unknown",
   "arch": "avr",
   "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
+  "max-atomic-width": 8,
 
   "executables": true,