tzmappings format has changed, breaking native-image timezone support in JDK 16

[shoaniki]

Describe the issue
The tzmappings file used to convert Windows timezone names to Java has changed format slightly, dropping an obsolete field. SVM includes a copy of the old code in order to read the file from memory instead of disk, but the experimental JDK 16 builds use this to parse the new file, with the result that it invariably reports "Illegal format in tzmappings file: illegal null character found at line 1, offset 47" instead of converting time zones.

Steps to reproduce the issue
Please include both build steps as well as run steps

1. Install a JDK 16 build of GraalVM 21.1.0 on Windows
2. Run the pre-packaged gu (or any other native-image binary that causes time zones to be looked up)

Describe GraalVM and your environment:

• GraalVM version: 21.1.0
• JDK major version: 16
• OS: Windows 10
• Architecture: AMD64

More details
The problematic code is in substratevm/src/com.oracle.svm.native.libchelper/src/timeZone.c.
It's pretty straightforward -- there used to be four fields per line and now there are three :)

(While debugging this problem I also became suspicious that SVM_readBufferUntilNewLine looks like it's skipping an extra byte before writing the null terminator -- I discovered the real cause before digging too far but it might be worth someone checking that out.)