[liba] Add aeabi-memclr4

Change-Id: I9ce3282773e2d1ada2f731e5c5c7ee07e7706612

liba/Makefile

@@ -7,4 +7,7 @@ tests += $(addprefix liba/test/, stdint.c)
 
 # The use of aeabi-rt could be made conditional to an AEABI target.
 # In practice we're always using liba on such a target.
-objs += $(addprefix liba/src/aeabi-rt/, memcpy.o)
+objs += $(addprefix liba/src/aeabi-rt/, \
+  memclr.o \
+  memcpy.o \
+)

liba/src/aeabi-rt/README.txt

@@ -9,6 +9,7 @@ memory copying/clearing/setting, etc...
 Since we're telling our compiler to build an AEABI binary, it may decide to use
 those symbols, and so we have to provide an implementation for them. Refer to
 the "Run-time ABI for the ARM Architecture" for a full list of functions.
+http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf
 
 Note that this is not formally the job of a libc implementation. Similar code is
 often shipped alongside a compiler (LLVM calls it compiler-rt, GCC libgcc). But

liba/src/aeabi-rt/memclr.c

@@ -0,0 +1,8 @@
+#include <stddef.h>
+#include <string.h>
+
+/* See the "Run-time ABI for the ARM Architecture", Section 4.3.3 */
+
+void __aeabi_memclr4(void * dest, size_t n) {
+  memset(dest, 0, n);
+}