Bump the minimum stack size to at least 1MB

[waahm7]

Issue #, if available:
awslabs/aws-crt-java#737

Description of changes:
Bump the minimum thread stack size to 1MB. Other Platforms have the following values

• Windows: 1MB
• GLIBC: 2MB - 8MB
• Musl 128KB (Used to be 80KB)
• Mac: 8MB for primary threads and 512KB for secondary threads.

I have tested a 1MB limit with the original customer issue sample, and it works.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.40%. Comparing base (852f8ce) to head (54de546).

Files	Patch %	Lines
source/posix/thread.c	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1139      +/-   ##
==========================================
- Coverage   83.44%   83.40%   -0.04%     
==========================================
  Files          57       57              
  Lines        5985     5995      +10     
==========================================
+ Hits         4994     5000       +6     
- Misses        991      995       +4     

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