Fix Linux available memory reporting to use MemAvailable instead of get_avphys_pages()

On Linux, get_avphys_pages() maps to sysconf(_SC_AVPHYS_PAGES) which returns
only completely unallocated pages (MemFree). The Linux kernel eagerly uses
spare physical RAM for disk caching (buff/cache). When active disk caching
fills memory, MemFree routinely hovers between 200-500 MB even when gigabytes
of memory are available.

This caused the viewer's memory budget logic to falsely report low system
memory (<256 MB), repeatedly triggering emergency texture downrezzing loops
and dynamic draw-distance halving.

Replace get_avphys_pages() in LLMemoryInfo::updateAvailableMemory() with a
fast, non-allocating single fread of MemAvailable: from /proc/meminfo
(available on Linux >= 3.14), with fallback to get_avphys_pages().

0 files changed, 0 insertions, 0 deletions