Implement __memset_chk as a copy & paste of __memcpy_chk.

These two will stay behind when we move memcpy()/memmove()/memset() over
to arm-optimized-routines (which leaves fortify to us).

Test: treehugger
Change-Id: Ie683f71a5a141263ce3f4e8811df9eaf667584f4
diff --git a/libc/Android.bp b/libc/Android.bp
index 6042929..5334763 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -787,7 +787,7 @@
     arch: {
         arm: {
             cflags: [
-                "-DNO___MEMCPY_CHK",
+                "-DHAVE_ASSEMBLER___MEMCPY_CHK",
                 "-DRENAME___STRCAT_CHK",
                 "-DRENAME___STRCPY_CHK",
             ],
@@ -814,9 +814,10 @@
             ],
         },
         arm64: {
-            cflags: ["-DNO___MEMCPY_CHK"],
+            cflags: ["-DHAVE_ASSEMBLER___MEMCPY_CHK"],
             srcs: [
                 "arch-arm64/generic/bionic/__memcpy_chk.S",
+                "arch-arm64/generic/bionic/__memset_chk.S",
             ],
         },
     },