crt: Fix __mingw_get_crt_info and remove broken revstamp.h

revstamp.h isn't autogenrated and is broken (and likely have been broken
since migrating from SVN to Git), so remove it.

Use __DATE__ and __TIME__ instead of __MINGW_W64_REV_STAMP in
__mingw_get_crt_info.

Accordingly changes Makefile.am.

Signed-off-by: Zhongteng Gui <dragon-archer@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>