Fix available memory extraction on Linux (dotnet/coreclr#26764)

Fix available memory extraction on Linux (dotnet/coreclr#26764)

* Fix available memory extraction on Linux

The GlobalMemoryStatusEx in PAL is returning number of free physical pages in
the ullAvailPhys member. But there are additional pages that are allocated
as buffers and caches that get released when there is a memory pressure and
thus they are effectively available too.

This change extracts the available memory on Linux from the /proc/meminfo
MemAvailable row, which is reported by the kernel as the most precise
amount of available memory.

Commit migrated from https://github.com/dotnet/coreclr/commit/859f464d867c5529d4fc5ca747751dea06895da4