[PLAT-8643] Fix a rare crash in BugsnagBreadcrumbsWriteCrashReport()

[nickdowell]

Goal

Fix a rare crash that has been observed in BugsnagBreadcrumbsWriteCrashReport().

A small number of crashes have been seen with the signature:

EXC_BAD_ACCESS

0 libsystem_platform.dylib _platform_strlen
1 Bugsnag                  bsg_kscrw_i_addJSONElement
2 Bugsnag                  BugsnagBreadcrumbsWriteCrashReport
3 Bugsnag                  BSSerializeDataCrashHandler
4 Bugsnag                  bsg_kscrashreport_writeStandardReport
5 Bugsnag                  ksmachexc_i_handleExceptions
6 libsystem_pthread.dylib  _pthread_start

... which appears to be due to BugsnagBreadcrumbsWriteCrashReport() accessing breadcrumbs that have been freed.

In theory this should not occur, because all possible writer threads should be suspended while this function runs, but it's possible that a call to thread_suspend fails or a thread is spawned after the thread list has been captured, leading to an active writer.

Changeset

Adds an atomic_bool to prevent memory being freed while BugsnagBreadcrumbsWriteCrashReport() is running, and makes g_breadcrumbs_head atomic to ensure it is fully stored before memory is freed.

Testing

Amends -testCrashReportWriterConcurrency to verify that BugsnagBreadcrumbsWriteCrashReport() is safe to call while writer threads are running.

[github-actions]

Bugsnag.framework binary size increased by 80 bytes from 815,832 to 815,912

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +72  +0.0%     +72    __TEXT,__text
  +0.0%     +48  +0.0%     +48    String Table
  +0.0%     +32  +0.0%     +32    Symbol Table
  [ = ]       0  +0.1%      +8    __DATA,__bss
  +0.2%      +8  +0.2%      +8    __TEXT,__gcc_except_tab
  +0.1%      +4  +0.1%      +4    __TEXT,__unwind_info
  [ = ]       0  -0.1%      -8    [__DATA]
  [ = ]       0  -2.4%     -80    [__LINKEDIT]
  -0.4%     -84  -0.4%     -84    [__TEXT]
  +0.0%     +80  [ = ]       0    TOTAL

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