Everybody gets what they want!
* AT_RANDOM is completely ignored.
* On Linux, MADV_WIPEONFORK is used to provide fork safety.
* On pthread systems, `pthread_atfork` is used to provide fork safety.
* For systems that do not have the capability to provide fork safety,
the implementation falls back to calling getrandom() every time.
* If madvise is unavailable or returns an error, or pthread_atfork
fails for whatever reason, it falls back to calling getrandom() every
time.
* Applications may choose to opt-out of fork safety.
* Applications may choose to opt-in to unconditionally calling
getrandom() for every call to std.crypto.random.fillFn.
* Added `std.meta.globalOption`.
* Added `std.os.madvise` and related bits.
* Bumped up the size of the main thread TLS buffer. See the comment
there for justification.
* Simpler hot path in TLS initialization.
ad05509 introduced a fix for the wrong problem, the logic to align the
start of main_thread_tls_buffer was already there but was flawed.
Fix it for good and avoid wasting too many bytes for alignment purposes.
Calling @panic made the executable ~30x times bigger, use a simple
`abort()` and let the user figure out what went wrong.
Supporting ARMv6 (and earlier?) platforms is not a priority.
Closes#6676
* Always allocate an info block per-thread so that libc can store
important stuff there.
* Respect ABI-mandated alignment in more places.
* Nicer code, use slices/pointers instead of raw addresses whenever
possible.
This reverts commit ee6fda2297bf75432b8d7115ec4c60c213535bbe, reversing
changes made to f313ab18aecea1ade0b6a90d671352a641ad351a.
This caused a test failure:
```
behavior.misc.test "behavior-arm-linux-none-Debug-bare-multi thread local variable"...test failure
/home/vsts/work/1/s/lib/std/testing.zig:191:14: 0x4608f in std.testing.expect (test)
if (!ok) @panic("test failure");
^
/home/vsts/work/1/s/test/stage1/behavior/misc.zig:616:11: 0x53e93 in behavior.misc.test "behavior-arm-linux-none-Debug-bare-multi thread local variable" (test)
expect(S.t == 1235);
^
```
* Always allocate an info block per-thread so that libc can store
important stuff there.
* Respect ABI-mandated alignment in more places.
* Nicer code, use slices/pointers instead of raw addresses whenever
possible.
