riscv64: add vector stpcpy().

Bug: https://github.com/google/android-riscv64/issues/107
Test: treehugger
Change-Id: Ic1fb9da974769d2767ea2c092a2e97e095612e6e
diff --git a/libc/Android.bp b/libc/Android.bp
index 68a0838..267fae2 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -951,6 +951,7 @@
                 "arch-riscv64/string/memcpy_vext.S",
                 "arch-riscv64/string/memmove_vext.S",
                 "arch-riscv64/string/memset_vext.S",
+                "arch-riscv64/string/stpcpy_vext.S",
                 "arch-riscv64/string/strcat_vext.S",
                 "arch-riscv64/string/strchr_vext.S",
                 "arch-riscv64/string/strcmp_vext.S",