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

sys/modules/linux64/Makefile

@@ -36,7 +36,7 @@ linux_locore.o: linux_locore.s linux_assym.h
 	    ${.IMPSRC} -o ${.TARGET}
 
 ${VDSO}.so: linux_locore.o
-	${OBJCOPY} --input-target binary --output-target elf64-x86-64-freebsd		\
+	${OBJCOPY} --input-target binary --output-target elf64-x86-64 \
 	    -S -g --binary-architecture i386:x86-64 linux_locore.o ${.TARGET}
 	strip -N _binary_linux_locore_o_size ${.TARGET}