lib: add stdlib.c

Since we need to realize standard library function other than use them
with gcc tool chain in U-Boot. So add standard library function here.

Change-Id: I10009c5bbe31fabacd929df3c4

lib: add stdlib.c

Since we need to realize standard library function other than use them
with gcc tool chain in U-Boot. So add standard library function here.

Change-Id: I10009c5bbe31fabacd929df3c44218ae9c6a885f
Signed-off-by: Jason Zhu <jason.zhu@rock-chips.com>

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