fixes #26015; Multiple definition error when using codegenDecl regression (#26018)

fixes #26015

Fixes imported global variables with codegenDecl being emitted as
definitions instead of extern declarations.

A variable’s codegenDecl format should customize its definition in the
owning module. Other modules referencing the variable must emit a normal
declaration:

```c
extern NI variable;
```

After the variable-declaration builder refactor, genVarPrototype passed
Extern visibility to addVar. However, the sfCodegenDecl branch returned
before applying that visibility. This caused importing modules to emit
another tentative definition, resulting in duplicate-symbol linker
errors.

The fix restores the previous distinction between the custom definition
and cross-module prototypes. It also adds C and C++ regression coverage
for both direct access and access through an inline procedure.

follows up https://github.com/nim-lang/Nim/pull/24423

compiler/cgen.nim

@@ -1854,10 +1854,16 @@ proc genVarPrototype(m: BModule, n: PNode) =
         typ = ptrType(typ)
       if lfDynamicLib in sym.loc.flags:
         typ = ptrType(typ)
-      m.s[cfsVars].addVar(m, sym,
-        name = sym.loc.snippet,
-        typ = typ,
-        visibility = vis)
+      if sfCodegenDecl in sym.flags:
+        m.s[cfsVars].addDeclWithVisibility(vis):
+          m.s[cfsVars].addVar(m, sym,
+            name = sym.loc.snippet,
+            typ = typ)
+      else:
+        m.s[cfsVars].addVar(m, sym,
+          name = sym.loc.snippet,
+          typ = typ,
+          visibility = vis)
       if m.hcrOn:
         m.initProc.procSec(cpsLocals).add('\t')
         m.initProc.procSec(cpsLocals).addAssignment(sym.loc.snippet,

tests/ccgbugs/mcodegendeclglobal.nim

@@ -0,0 +1,4 @@
+var codegenDeclGlobal* {.codegenDecl: "$# /* custom declaration */ $#".} = 123
+
+proc readCodegenDeclGlobal*(): int {.inline.} =
+  codegenDeclGlobal

tests/ccgbugs/tcodegendeclglobal.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''
+123
+123
+'''
+  ccodecheck: "'extern NI /* custom declaration */ codegenDeclGlobal'"
+  targets: "c cpp"
+"""
+
+import ./mcodegendeclglobal
+
+echo codegenDeclGlobal
+echo readCodegenDeclGlobal()