asm-generic: current: Don't include thread-info.h if building asm

asm/current.h is included by some assembly files (either directly, or
indirectly). This works on some architectures (such as x86), as their
implementation of current.h is careful to not include any C, but the
asm-generic version includes linux/thread-info.h unconditionally, which
leads to errors when either C code or non-asm-compatible preprocessor
directives are included.

Just wrap the contents behind an #ifndef __ASSEMBLY__ to avoid any C
code making its way in.

Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Richard Weinberger <richard@nod.at>