[git] Add gitattributes to convert CRLF to LF automatically

This attribute will apply to newly added file only. If they contain
CRLF, this will be converted to LF automatically.

R=mathias@chromium.org

Bug: v8:7570
Change-Id: Idbf1f15d55dda18f623f9b3c7be8bba686569af0
Reviewed-on: https://chromium-review.googlesource.com/1047608
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53044}

.gitattributes

+# Automatically normalize line endings (to LF) for all text-based files.
+* text=auto