Better support for QMAKE_POST_LINK for symbian-sbsv2 builds.

If QMAKE_POST_LINK statement is needed for a project, currently it is almost always necessary to have a separate QMAKE_POST_LINK statement for symbian-sbsv2 while other mkspecs often can use one generic statement. This is because $$QMAKE_COPY, $$QMAKE_DIR_SEP, and the like variables resolve to Windows counterparts when mkspec is symbian-sbsv2 and the host OS is Windows, but sbsv2 toolchain requires Unix equivalents to be used as it utilizes Cygwin. This cannot be simply fixed by changing the definitions of these variables, as the Windows ones are needed when wrapper Makefile is generated.

For QMAKE_EXTRA_COMPILERS this problem is already resolved by doing smart replacement of various commands and blanket replacement of blackslashes to slashes. Similar replacements should be done for QMAKE_POST_LINK variable. Note that while blanket backslash to slash replacement seems dangerous, it should be safe as sbsv2 toolchain strips all backslashes from all bld.inf extension OPTION parameters anyway, so they can't be used in the first place.