Revert "Switch back to the BSD exec for now."

This reverts commit 18c31d7f4c93309a0f316412fcc6a829e41eaaae.

63615066d9f0b0b1c653c91a9b305e6c292c835a fixed the bug.

Change-Id: I1d59c4be6ad1a05b23dce0198737f0789d7e034a
diff --git a/libc/Android.bp b/libc/Android.bp
index 52006a4..4fcd83e 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -518,7 +518,6 @@
     name: "libc_openbsd_large_stack",
     defaults: ["libc_defaults"],
     srcs: [
-        "upstream-openbsd/lib/libc/gen/exec.c",
         "upstream-openbsd/lib/libc/stdio/vfprintf.c",
         "upstream-openbsd/lib/libc/stdio/vfwprintf.c",
         "upstream-openbsd/lib/libc/stdio/vfwscanf.c",
@@ -1238,6 +1237,7 @@
         "bionic/error.cpp",
         "bionic/eventfd_read.cpp",
         "bionic/eventfd_write.cpp",
+        "bionic/exec.cpp",
         "bionic/faccessat.cpp",
         "bionic/fchmod.cpp",
         "bionic/fchmodat.cpp",