Move sysroots into subdirectory

Move the sysroots into a subdirectory so they don't get confused with
binary prebuilts.

Test: llvm build
Change-Id: I7b8384282ccaa0cc92208c061b99c0ea99fbd315
diff --git a/update-prebuilts.sh b/update-prebuilts.sh
index 4f87b39..0ac14c2 100755
--- a/update-prebuilts.sh
+++ b/update-prebuilts.sh
@@ -57,8 +57,8 @@
 unzip_to linux-x86 "${tmpdir}/linux.zip"
 unzip_to common "${tmpdir}/common.zip"
 unzip_to darwin-x86 "${tmpdir}/darwin.zip"
-unzip_to x86_64-linux-musl "${tmpdir}/musl-sysroot64.zip"
-unzip_to i686-linux-musl "${tmpdir}/musl-sysroot32.zip"
+unzip_to sysroots/x86_64-linux-musl "${tmpdir}/musl-sysroot64.zip"
+unzip_to sysroots/i686-linux-musl "${tmpdir}/musl-sysroot32.zip"
 
 cp -f "${tmpdir}/manifest.xml" manifest.xml