arrow

ARROW-7464: [C++] Refine CpuInfo singleton with std::call_once

CpuInfo singleton is created and initialized on first invocation of
[CpuInfo::GetInstance()](https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/cpu_info.cc#L188-L195). All calls afterwards return reference to the
same instance. Current code uses std::mutex to make sure that CpuInfo
is created only once, but it introduces unnecessary overhead for later
calls. Concurrent threads getting the created instance should not block
each other.

Replace std::mutex with std::call_once to fix this issue.

References:
[1] https://en.cppreference.com/w/cpp/thread/call_once
[2] http://www.modernescpp.com/index.php/thread-safe-initialization-of-data

Closes #6091 from cyb70289/callonce and squashes the following commits:

f98632dda <Yibo Cai> Rename cpuinfo_once to cpuinfo_initialized
427579b5f <Yibo Cai> Fix lint issue
b75cd43b9 <Yibo Cai> ARROW-7464: : Refine CpuInfo singleton with std::call_once

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>