arch/alpha/kernel/vmlinux.lds.S uses it:

arch/alpha/kernel/vmlinux.lds:241: undefined symbol `THREAD_SIZE' referenced in expression

Seems to have been caused by

commit 9d93f005
Author:     Geoffrey Thomas <geofft@ksplice.com>
AuthorDate: Thu Sep 24 10:36:26 2009 -0400
Commit:     Linus Torvalds <torvalds@linux-foundation.org>
CommitDate: Thu Sep 24 17:16:22 2009 -0700

    alpha: Clean up linker script using new linker script macros.

    Note that .data.page_aligned and .data.cacheline_aligned are now after
    _data; it was probably a bug that they were before it.

    Also, some explicit ALIGN(8)'s between various initcall sections were
    removed; this should be harmless as the implicit alignment of
    initcall_t was already 8.

Cc: Geoffrey Thomas <geofft@ksplice.com>
Cc: Tim Abbott <tabbott@ksplice.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>