[PLAT-6113] Fix possible deadlock in bsg_ksmachfreeMemory

[nickdowell]

Goal

Fixes a deadlock that can occur because the implementation of __builtin_available() uses malloc() and is therefore not async-signal-safe.

This was observed when investigating potential causes of deadlocks in the crash handling code.

Design / Changeset

The availability of os_proc_available_memory() is now determined at initialization time rather than in the implementation of bsg_ksmachfreeMemory()

A function pointer is used to avoid the need for #pragma clang diagnostic statements in bsg_ksmachfreeMemory()

Testing

Manually verified by running example apps and adding log statements to confirm that os_proc_available_memory() is used on the relevant platforms.

Added an E2E scenario that could consistently produce a deadlock in bsg_ksmachfreeMemory() prior to the fix, and confirmed that it passes with the fix in place.

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 104 bytes from 1,061,720 to 1,061,824

Generated by 🚫 Danger

[kattrali]

nicely done 🎉