Add libstd signal patch and patching script. am: cc7d3430f2 am: 988c2641f1 am: f7d3019987

Original change: https://android-review.googlesource.com/c/platform/prebuilts/rust/+/1446776

Change-Id: Ib8e5c58501e1da142dae23086f178bf180f1d03e
tree: 671fac5d9e5c6e72f71715c62b54f7a7fb8d1457
  1. bootstrap/
  2. darwin-x86/
  3. linux-x86/
  4. Android.bp
  5. OWNERS