Only use --whole-archive with gcc.

Conditionalize use of --whole-archive on the platform plus compiler,
rather than on the ABI.  This fixes a regression caused by
7b24c6e557 (Use --whole-archive when
linking integration tests on MinGW.).

msvc/ReadMe.txt

@@ -17,7 +17,7 @@ How to build jemalloc for Windows
    (note: x86/x64 doesn't matter at this point)
 
 5. Generate header files:
-   sh -c "./autogen.sh" CC=cl --enable-lazy-lock=no
+   sh -c "CC=cl ./autogen.sh"
 
 6. Now the project can be opened and built in Visual Studio:
    msvc\jemalloc_vc2015.sln