[roll] Roll fuchsia [build][profile] Linux-native system_profiler

Overhauls the background resource profiling system used by 'fx build-profile'
and 'fx build' by implementing a pure, zero-subprocess Python daemon
called 'system_profiler.py'.

The new daemon polls `/proc` filesystem directly on an interval and
collects advanced performance and load metrics:
- CPU ticks and state percentages from /proc/stat
- Thread states and active tasks from /proc/stat
- Detailed memory and Dirty/Writeback cache sizes from /proc/meminfo
- Paging rates and blocks read/written from /proc/vmstat
- Pressure Stall Information (PSI) for CPU, Mem, and I/O from /proc/pressure
- System load averages from /proc/loadavg
- Network ingress/egress bytes/packets per-interface and aggregate from /proc/net/dev
- System-wide and process-tree open file descriptor counts

All parsing is decoupled from I/O and written as pure module-scope
functions returning immutable frozen dataclasses.

Each sampler is implemented as its own class, and plugged into
the generic SystemProfiler engine.

Output is streamed to file as a JSON-array Chrome trace.

Because the direct `/proc` implementation is Linux-specific, also update
tools/devshell/lib/vars.sh to only default-enable build profiling when
the host platform is Linux.

Original-Original-Bug: 515107639
Test: fx test system_profiler_test
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1636716
Original-Original-Revision: 4dc2b6bce54955d5de7f09db85b0a7f528a2db32
GitOrigin-RevId: 2844ec039754d913201006d846d9718455ef8354
Change-Id: Iab8f90110a9c46b71be4b74359dd9fa371c7e007
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1638393
Cr-Commit-Position: refs/heads/main@{#194018}