Because the Red Hat Enterprise Linux 8 core repository does not include
libpmem, let us implement the necessary subset ourselves.

pmem_persist(): Implement for 64-bit x86, ARM, POWER, RISC-V, Loongarch
in a way that should be compatible with the https://github.com/pmem/pmdk/
implementation of pmem_persist().

The CMake option WITH_INNODB_PMEM can be used for enabling or disabling
this interface at compile time. By default, it is enabled on all applicable
systems that are covered by our CI system.

Note: libpmem had not been previously enabled for Loongarch in our
Debian packaging. It was enabled for RISC-V, but we will not enable it
by default on RISC-V or Loongarch because we lack CI coverage.

The generated code for x86_64 was reviewed and tested on two
Intel implementations: one that only supports clflush, and
another that supports both clflushopt and clwb.

The generated machine code was also reviewed on https://godbolt.org
using various compiler versions. Godbolt helpfully includes an option
to compile to binary code and display the encoding, which was
useful on POWER.

Reviewed by: Vladislav Vaintroub