implemented alignas pragma (#12643)

* implemented alignas pragma

* fix bootstrap

* generate c++ compatible syntax for alignas

* Make it work.

* Multiple alignof expressions. Implement top level alignof.

tests/misc/talignas.nim

@@ -0,0 +1,46 @@
+discard """
+ccodeCheck: "\\i @'alignas(128) NI mylocal1' .*"
+target: "c cpp"
+output: "alignas ok"
+"""
+
+# This is for Azure. The keyword ``alignof`` only exists in ``c++11``
+# and newer. On Azure gcc does not default to c++11 yet.
+when defined(cpp) and not defined(windows):
+  {.passC: "-std=c++11".}
+
+import globalalignas
+
+var toplevel1 {.alignas: 32.} : int32
+var toplevel2 {.alignas: 32.} : int32
+var toplevel3 {.alignas: 32.} : int32
+
+proc foobar() =
+  var myvar1 {.global, alignas(64).}: int = 123
+  var myvar2 {.global, alignas(64).}: int = 123
+  var myvar3 {.global, alignas(64).}: int = 123
+
+  doAssert (cast[uint](addr(myglobal1)) and 127) == 0
+  doAssert (cast[uint](addr(myglobal2)) and 127) == 0
+  doAssert (cast[uint](addr(myglobal3)) and 127) == 0
+
+  doAssert (cast[uint](addr(myvar1)) and 63) == 0
+  doAssert (cast[uint](addr(myvar2)) and 63) == 0
+  doAssert (cast[uint](addr(myvar3)) and 63) == 0
+
+  doAssert (cast[uint](addr(toplevel1)) and 31) == 0
+  doAssert (cast[uint](addr(toplevel2)) and 31) == 0
+  doAssert (cast[uint](addr(toplevel3)) and 31) == 0
+
+  # test multiple alignas expressions
+  var mylocal1 {.alignas(0), alignas(128), alignas(32).}: int = 123
+  var mylocal2 {.alignas(128), alignas(0), alignas(32).}: int = 123
+  var mylocal3 {.alignas(0), alignas(32), alignas(128).}: int = 123
+
+  doAssert (cast[uint](addr(mylocal1)) and 127) == 0
+  doAssert (cast[uint](addr(mylocal2)) and 127) == 0
+  doAssert (cast[uint](addr(mylocal3)) and 127) == 0
+
+  echo "alignas ok"
+
+foobar()