Fix back-slash in archive action.

Unlike all the other actions in gcc, the archive action was producing platform paths instead of regular slash paths. The platform paths on Windows (\) cause problems when placed in response files as gcc/mingw interpret them as escape markers. This adds the :T qualifier to normalize the slashes.

fixes #324 
fixes #352 
fixes boostorg/url#779

src/tools/gcc.jam

@@ -1091,7 +1091,7 @@ rule archive ( targets * : sources * : properties * )
 #
 actions piecemeal archive
 {
-    "$(.AR)" $(AROPTIONS) $(.ARFLAGS) "$(<)" @($(<[-1]:T).rsp:O=FC:<=@":>=":E="$(>)")
+    "$(.AR)" $(AROPTIONS) $(.ARFLAGS) "$(<)" @($(<[-1]:T).rsp:O=FC:<=@":>=":E="$(>:T)")
 }
 
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