x86_64: fix arch_vma_name

The function arch_vma_name() is declared weak and thus it was
not noticed that x86_64 had two almost identical implementations.

It was introduced in syscall32.c by: c633090e3105e779c97d4978e5e3d7d66b291cfb
It was introduced in mm/init.c by: 2aae950b21e4bc789d1fc6668faf67e8748300b7

Signed-off-by: Roland McGrath <roland@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>