tools: fix error when `/bin/sh` isn't `bash`

The following scripts use `bash` specific variable `${BASH_SOURCE[0]}`
so this doesn't work correct in case when `/bin/sh` isn't `bash`.

Test: repo upload bionic
Change-Id: I12ce0666f9404107dceb03960bad4ec11b14ba2d
Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>
diff --git a/tools/update_syscalls.sh b/tools/update_syscalls.sh
index 3ad95ed..5e7eb0a 100755
--- a/tools/update_syscalls.sh
+++ b/tools/update_syscalls.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 cd $DIR/..
 ./libc/tools/gensyscalls.py