gcc/mingw: Align `.refptr.` to 8-byte boundaries for 64-bit targets

gcc/mingw: Align `.refptr.` to 8-byte boundaries for 64-bit targets

Windows only requires sections to be aligned on a 4-byte boundary. This used
to work because in binutils the `.rdata` section is over-aligned to a 16-byte
boundary, which will be fixed in the future.

This matches the output of Clang.

Signed-off-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>
gcc/ChangeLog:
	* config/mingw/winnt.cc (mingw_pe_file_end): Add `.p2align`.