Fix #21241 - ImportC: wrong static function called after linking (#21242)

Fixes https://github.com/dlang/dmd/issues/21241

C Static functions were being given the same externally mangled
name as their identifier, which caused only one to be picked
when linking with linkers that supported that.

Additionally, the dmd glue code was only outputting one of these
static functions as a workaround for a different linker issue.

Solve this by giving C static functions a unique name (by using D
mangling) and adding an `isStatic()` check to the dmd glue hack.

compiler/test/runnable/imports/imp21241a.c

@@ -0,0 +1,9 @@
+// https://github.com/dlang/dmd/issues/21241
+enum {aValue=1};
+static int foo(void){
+    return aValue;
+}
+
+int getA(void){
+    return foo();
+}

compiler/test/runnable/imports/imp21241b.c

@@ -0,0 +1,9 @@
+// https://github.com/dlang/dmd/issues/21241
+enum {bValue=2};
+static int foo(void){
+    return bValue;
+}
+
+int getB(void){
+    return foo();
+}

compiler/test/runnable/test21241.d

@@ -0,0 +1,13 @@
+/*
+REQUIRED_ARGS: runnable/imports/imp21241a.c runnable/imports/imp21241b.c
+*/
+// https://github.com/dlang/dmd/issues/21241
+import imp21241a;
+import imp21241b;
+
+void main(){
+    int x = getA();
+    assert(x==aValue);
+    x = getB();
+    assert(x==bValue);
+}