Compile stdlib C files individually

All C files of stdlib were included into std.c, which was the file
that the Makefile actually compiled. This is a poor way of compiling
all the files and, while it may work fine most times, it's
discouraged.

In this particular case, each C file included its own headers, which
were later included into std.c. For example, this caused problems
because a duplicated typedef of u_short in both subr_prf.c and
types.h. While that may require an issue on its own, this kind of
problems are avoided if all C files are as independent as possible.

Change-Id: I9a7833fd2933003f19a5d7db921ed8542ea2d04a
diff --git a/Makefile b/Makefile
index ac688ba..81991b8 100644
--- a/Makefile
+++ b/Makefile
@@ -197,7 +197,19 @@
 				lib/aarch64/cache_helpers.S		\
 				lib/aarch64/misc_helpers.S		\
 				lib/aarch64/xlat_helpers.c		\
-				lib/stdlib/std.c			\
+				lib/stdlib/abort.c			\
+				lib/stdlib/assert.c			\
+				lib/stdlib/exit.c			\
+				lib/stdlib/mem.c			\
+				lib/stdlib/printf.c			\
+				lib/stdlib/putchar.c			\
+				lib/stdlib/puts.c			\
+				lib/stdlib/sscanf.c			\
+				lib/stdlib/strchr.c			\
+				lib/stdlib/strcmp.c			\
+				lib/stdlib/strlen.c			\
+				lib/stdlib/strncmp.c			\
+				lib/stdlib/subr_prf.c			\
 				plat/common/aarch64/platform_helpers.S
 
 INCLUDES		+=	-Iinclude/bl1			\