Port abseil to GNU/Hurd.

[crupest]

This pr is to port abseil to GNU/Hurd. It might be a little hard for you to thoroughly review and test the patch on GNU/Hurd manually. But feel free to talk about any problems on it. I'm here willing to explain as much as I can. Here are some info that might be helpful.

• I have succeeded to package it to .deb on GNU/Hurd Debian distribution. One step of the .deb packaging process is to run the unit tests, just like Debian on other platforms. And they passed.

• I have recently submitted a patch to libgav1, which is the av1 decoder used in Chromium (maybe), and depends on abseil. I fixed its compiling failure on GNU/Hurd and built it with ported abseil. It works well and I can use it to decode real av1 videos.

• This patch was firstly posted to GNU/Hurd Debian mailing list for review, and some of the maintainers of GNU/Hurd have checked it. Some discussions also took place on irc channel. If you are interested, check here [1] [2] [3].

Following is the commit message. Most platform technical details are described on it.

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abseil has failed to build on GNU/Hurd for a long time. Now let's make it work! :)

Note that __GNU__ is the macro for detecting GNU/Hurd. And __MACH__ is also defined there besides on Apple platform. They are both "mach" but with different implementation and platform details. Here are the works,

• Mark platform features (not) supported by GNU/Hurd.

  • Supports mmap and write.

  • Not supports vdso. It's specific to Linux.

  • Not supports ELF_SYMBOLIZE for now. GNU/Hurd uses ELF as the binary format. But symbolizing in abseil relies on reading object file path from /proc/self/task/<pid>/maps (Linux specific) or /proc/self/maps. GNU/Hurd does have the latter. However, due to its micro-kernel design, it's currently unable to get the file path info there.

  • Disable stack consumption measurement. The problem behind it is that GNU/Hurd uses a very different way to implement signal handling. Due to compiler behavior, it is impossible to get a stable, valid and reliable statictic data. In my test environment, it's 96 bytes (< 100) for current codes.

• GNU/Hurd uses different errno and messages than Linux. So related things are adjusted accordingly.

• Fix a misuse of __MACH__, which should actually be __APPLE__.

• Fix a missing including of signal.h for using SIGABRT. Otherwise compilation will fail with undefined symbol error on GNU/Hurd.