inc and dec are atomic in JS backend (#16129)

lib/system.nim

@@ -1774,7 +1774,7 @@ when not defined(nimscript):
 when not declared(sysFatal):
   include "system/fatal"
 
-when notJSnotNims:
+when not defined(nimscript):
   {.push stackTrace: off, profiler: off.}
 
   proc atomicInc*(memLoc: var int, x: int = 1): int {.inline,

tests/system/tatomics1.nim

@@ -0,0 +1,9 @@
+discard """
+  target: "c cpp js"
+"""
+
+var x = 10
+atomicInc(x)
+doAssert x == 11
+atomicDec(x)
+doAssert x == 10