Incorrect glibc version is used in a comment in os_linux.cpp

From JDK-8169373:

The glibc NPTL bug which subtracted the guard size from the thread stack size instead of adding it on top of it has been fixed in glibc 2.27 (see https://sourceware.org/bugzilla/show_bug.cgi?id=22637). This leads to slightly bigger stack sizes for non-Java threads if running an glibc 2.27+ but I think the effect is negligible and only affects non-Java threads because Java threads don't use glibc guard pages.

"JDK-8225035: Thread stack size issue caused by large TLS size" mentioned this in a comment (https://github.com/openjdk/jdk/blob/299022dfacbcb49e3bc5beca8ff9b1fca1101493/src/hotspot/os/linux/os_linux.cpp#L809) but used the wrong glibc version "2.7" instead of "2.27".

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