use ASM_NL instead of '; ' for assembler new line character in the macro

For some assemblers, they use another character as newline in a macro
(e.g. arc uses '`'), so for generic assembly code, need use ASM_NL (a
macro) instead of ';' for it.

Basically this is the same patch as applied to Linux kernel -
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/include/linux/linkage.h?id=9df62f054406992ce41ec4558fca6a0fa56fffeb



but modified a bit to fit in U-Boot.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>