Use Linux's prctl(PR_SET_VMA) system call to name memory arenas being
allocated using mmap(MAP_ANONYMOUS).

This change allows Abseil's memory arena(s) to be distinguished from
other uses of anonymous memory within a process, which in turn helps
investigations into the memory usage of applications.

The change adds a new prctl() system call to the code paths that call
mmap(). This is not expected to add significant overhead to applications.

The call to prctl(PR_SET_VMA, ...) can fail if the Linux kernel in use was
not configured with the CONFIG_ANON_VMA_NAME kernel option.  This should
be OK since the naming memory regions is primarily a debugging aid.

PiperOrigin-RevId: 523687348
Change-Id: Ie404e5eeef0a6da53330b3a56149c4f3bc6bf5c7