Enable macros to use certain things from the OS module when the target OS is not supported (#24639)

Essentially this PR removes the `{.error.}` pragmas littered around in
the OS module and submodules which prevents them from being imported if
the target OS is not supported. This made it impossible to use certain
supported features of the OS module in macros from a supported host OS.
Instead of the `{.error.}` pragmas the `oscommon` module now has a
constant `supportedSystem` which is false in the cases where the
`{.error.}` pragmas where generated. All procedures which can't be run
by macros is also not declared when `supportedSystem` is false.

It would be possible to create dummy versions of the omitted functions
with an `{.error.}` pragma that would trigger upon their use, but this
is currently not done.

This properly fixes #19414

Author: PMunch

compiler/vmops.nim

@@ -263,7 +263,7 @@ proc registerAdditionalOps*(c: PCtx) =
     wrap2si(readLines, ioop)
     systemop getCurrentExceptionMsg
     systemop getCurrentException
-    registerCallback c, "stdlib.osdirs.staticWalkDir", proc (a: VmArgs) {.nimcall.} =
+    registerCallback c, "stdlib.staticos.staticWalkDir", proc (a: VmArgs) {.nimcall.} =
       setResult(a, staticWalkDirImpl(getString(a, 0), getBool(a, 1)))
     registerCallback c, "stdlib.staticos.staticDirExists", proc (a: VmArgs) {.nimcall.} =
       setResult(a, dirExists(getString(a, 0)))