traceprof: enforce native stderr on Windows to avoid crash

Summary:
On Windows `FILE*` seems to be different in traceprofimpl.cpp and Python:

```
# fp passed from traceprof.pyx:profile(fp=sys.stderr) to traceprofimpl.cpp
0x0000000000d23cc0 {_Placeholder=0x0000000000d23360 }
    _Placeholder: 0x0000000000d23360

# stderr as seen from traceprofimpl.cpp natively
ucrtbase.dll!0x00007ffb29465510 (load symbols for additional information) {_Placeholder=0x0000000000000000 }
    _Placeholder: 0x0000000000000000
```

Workaround that by enforcing output Windows to use stderr for now.

Test Plan:
```
C:\hg> python2 setup.py build_ext -i
...

C:\hg> type a.py
import sys
from mercurial import ui
from hgext import traceprof
def f(): g(1)
def g(x): print(x)
with traceprof.profile(ui.ui(), sys.stderr): f()

C:\hg> python2 a.py
1
    | <module>                                  a.py:1
    | __exit__                                  C:\...\contextlib.py:21
Total time: 0 ms
```

Reviewers: #mercurial

Differential Revision: https://phabricator.intern.facebook.com/D6686958

hgext/traceprof.pyx

@@ -36,7 +36,10 @@ import gc
 cdef extern from "extlib/traceprofimpl.cpp":
     void enable()
     void disable()
-    void report(FILE *)
+    IF UNAME_SYSNAME == "Windows":
+        void report()
+    ELSE:
+        void report(FILE *)
     void settimethreshold(double)
     void setcountthreshold(size_t)
     void setdedup(int)
@@ -63,5 +66,10 @@ def profile(ui, fp):
         yield
     finally:
         disable()
-        report(PyFile_AsFile(<PyObject *>fp))
+        IF UNAME_SYSNAME == "Windows":
+            # On Windows, "FILE*" could be incompatible between Python.exe and
+            # traceimpl.exp. Use stderr from traceimpl.exp conservatively.
+            report()
+        ELSE:
+            report(PyFile_AsFile(<PyObject *>fp))
         clear()