gendef: Do not set stdcall @0 for non-stdcall functions parsed from existing def files

If there is a def file "lib1.def" with line "func1" and gendef was called
on lib2.dll which contains func2 symbol defined as PE forward to lib1.func1
then gendef produces "lib2.def" file with line "func2@0 = lib1.func1" even
when gendef was not called with -a option.

This behavior is wrong because func1 in lib1.def is not defined as stdcall
therefore gendef should not add stdcall decoration for func2.

This change fixes this issue, changes the way how gendef reads and parse
input def files.

Signed-off-by: LIU Hao <lh_mouse@126.com>