more checking for --gc:arc, no need for valgrind (#14467)

* sigmatch: removed dead code

lib/system/bitmasks.nim

@@ -0,0 +1,30 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2015 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+# Page size of the system; in most cases 4096 bytes. For exotic OS or
+# CPU this needs to be changed:
+const
+  PageShift = when defined(cpu16): 8 else: 12 # \
+    # my tests showed no improvements for using larger page sizes.
+  PageSize = 1 shl PageShift
+  PageMask = PageSize-1
+
+  MemAlign = 16 # also minimal allocatable memory block
+
+  BitsPerPage = PageSize div MemAlign
+  UnitsPerPage = BitsPerPage div (sizeof(int)*8)
+    # how many ints do we need to describe a page:
+    # on 32 bit systems this is only 16 (!)
+
+  TrunkShift = 9
+  BitsPerTrunk = 1 shl TrunkShift # needs to be power of 2 and divisible by 64
+  TrunkMask = BitsPerTrunk - 1
+  IntsPerTrunk = BitsPerTrunk div (sizeof(int)*8)
+  IntShift = 5 + ord(sizeof(int) == 8) # 5 or 6, depending on int width
+  IntMask = 1 shl IntShift - 1