bring back std/ prefix within compiler and ensure it works in bootstrap + bsd (#17902)

* [WIP] bring back std/ prefix within compiler and ensure it works in bootstrap + bsd

* refs https://github.com/nim-lang/Nim/pull/16282#discussion_r616846863
* sounds very similar to https://github.com/nim-lang/Nim/pull/14291
* more: vmops
* update tools/ci_generate.nim
* auto-generate freebsd.yml as well, to avoid duplication with openbsd.yml
* cleanup
* undo temporary CI removal

compiler/ic/dce.nim

@@ -9,7 +9,7 @@
 
 ## Dead code elimination (=DCE) for IC.
 
-import intsets, tables
+import std/[intsets, tables]
 import ".." / [ast, options, lineinfos, types]
 
 import packed_ast, ic, bitabs

compiler/vmops.nim

@@ -9,24 +9,24 @@
 
 # Unfortunately this cannot be a module yet:
 #import vmdeps, vm
-from math import sqrt, ln, log10, log2, exp, round, arccos, arcsin,
+from std/math import sqrt, ln, log10, log2, exp, round, arccos, arcsin,
   arctan, arctan2, cos, cosh, hypot, sinh, sin, tan, tanh, pow, trunc,
   floor, ceil, `mod`, cbrt, arcsinh, arccosh, arctanh, erf, erfc, gamma,
   lgamma
 
 when declared(math.copySign):
-  from math import copySign
+  from std/math import copySign
 
 when declared(math.signbit):
-  from math import signbit
+  from std/math import signbit
 
-from os import getEnv, existsEnv, dirExists, fileExists, putEnv, walkDir,
+from std/os import getEnv, existsEnv, dirExists, fileExists, putEnv, walkDir,
                    getAppFilename, raiseOSError, osLastError
 
-from md5 import getMD5
-from times import cpuTime
-from hashes import hash
-from osproc import nil
+from std/md5 import getMD5
+from std/times import cpuTime
+from std/hashes import hash
+from std/osproc import nil
 
 from sighashes import symBodyDigest