RFR: JDK-8257547: Handle multiple prereqs on the same line in deps files

[Erik Joelsson]

After fixing JDK-8256810 and starting to look into backporting it, I discovered more potential failing cases. Certain versions of GCC may sometimes output multiple prerequisite files on the same line. I think the easiest way to handle this new issue is to split such lines.

When splitting lines, we need to make sure to not just split on any space. Some compilers output the : of the rule with a leading space, and already split lines will have a space before the backslash.

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Commit messages:
 - Handle multiple prereqs on the same line

Changes: https://git.openjdk.java.net/jdk/pull/1548/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1548&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257547
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1548.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1548/head:pull/1548

PR: https://git.openjdk.java.net/jdk/pull/1548