linux64: use linux output target for linux_vdso.so

linux_vdso.so provides the vdso for the linuxulator's amd64 target and
is mapped into a Linux binary's address space. Thus it should be a
Linux-style .so, which has the ELF OS/ABI unset.

It turns out that ELF Tool Chain elfcopy/objcopy also has a bug where
the OS/ABI field is unset, regardless of the specified --output-target,
so this change is a no-op with the default in-tree toolchain. This is a
real fix when using external binutils, and the ELF Tool Chain bug will
be fixed in the future.

PR: 228934
Sponsored by: Turing Robotic Industries