windows fastcall (x64) call convention (#314)

* initial set of work for windows fastcall (x64) call convention

- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
  The shadow space available before the arguments (range 0..32)
  is not used as spill space yet.

* address review feedback

lib/codegen/meta/base/settings.py

@@ -36,14 +36,22 @@ call_conv = EnumSetting(
         - fast: not-ABI-stable convention for best performance
         - cold: not-ABI-stable convention for infrequently executed code
         - system_v: System V-style convention used on many platforms
-        - fastcall: Windows "fastcall" convention, also used for x64 and ARM
+        - windows_fastcall: Windows "fastcall" convention, also used for
+                            x64 and ARM
         - baldrdash: SpiderMonkey WebAssembly convention
         - probestack: specialized convention for the probestack function
 
         The default calling convention may be overridden by individual
         functions.
         """,
-        'fast', 'cold', 'system_v', 'fastcall', 'baldrdash', 'probestack')
+
+        'fast',
+        'cold',
+        'system_v',
+        'windows_fastcall',
+        'baldrdash',
+        'probestack'
+)
 
 # Note that Cretonne doesn't currently need an is_pie flag, because PIE is just
 # PIC where symbols can't be pre-empted, which can be expressed with the