[flang] add QSORT extension intrinsic to the runtime (#132033)

Add support for legacy Fortran intrinsic QSORT from lib 3f.
This is a thin Fortran wrapper over libc qsort.

Author: jeanPerier

flang-rt/lib/runtime/extensions.cpp

@@ -20,6 +20,7 @@
 #include <cstring>
 #include <ctime>
 #include <signal.h>
+#include <stdlib.h>
 #include <thread>
 
 #ifdef _WIN32
@@ -262,5 +263,10 @@ int RTNAME(Chdir)(const char *name) {
 
 int FORTRAN_PROCEDURE_NAME(ierrno)() { return errno; }
 
+void FORTRAN_PROCEDURE_NAME(qsort)(int *array, int *len, int *isize,
+    int (*compar)(const void *, const void *)) {
+  qsort(array, *len, *isize, compar);
+}
+
 } // namespace Fortran::runtime
 } // extern "C"

flang/docs/Intrinsics.md

@@ -1106,3 +1106,34 @@ end program chdir_func
 - **Standard:** GNU extension
 - **Class:** function
 - **Syntax:** `RESULT = IERRNO()`
+
+### Non-Standard Intrinsics: QSORT
+
+#### Description
+
+```
+SUBROUTINE QSORT(ARRAY, LEN, ISIZE, COMPAR)
+  TYPE(*) :: ARRAY(*)
+  INTEGER(4) :: LEN, ISIZE
+  INTERFACE
+    INTEGER(4) FUNCTION COMPAR(A, B)
+      TYPE(*) :: A, B
+    END FUNCTION
+  END INTERFACE
+END SUBROUTINE
+```
+
+Sort `ARRAY` in place in ascending order given the comparison function `COMPAR`.
+The array number of elements is given by `LEN` and the element byte size is given
+by `ISIZE`.
+
+`COMPAR` function takes the addresses of element `A` and `B` and must return:
+- a negative value if `A` < `B`
+- zero if `A` == `B`
+- a positive value otherwise. 
+
+#### Usage and Info
+
+- **Standard:** lib3f (section 3f of old man pages).
+- **Class:** subroutine
+- **Syntax:** `CALL QSORT(ARRAY, LEN, ISIZE, COMPAR)`