uclibc++: add hack to fix failing patch

One of the patched files, include/unwind-cxx.h, contains windows newlines
which lead to the following failure:

  Applying ./patches/006-eabi_fix.patch using plaintext:
  patching file include/typeinfo
  patching file include/unwind-cxx.h
  Hunk #1 FAILED at 173 (different line endings).
  Hunk #2 FAILED at 181 (different line endings).

Add a fixup command to the prepare phase which normalizes the line endings
before applying source patches.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>