Fix _FORTIFY_SOURCE=2 runtime failure and enable security hardening features

[cclerget]

Description of the Pull Request (PR):

• Move child stack in the BSS section, stack usage seems is around 80 bytes (the actual stack size is unchanged: 4096)
• Switch buildmode=default to buildmode=pie to leverage kernel ASLR
• Add _FORTIFY_SOURCE=2 and fstack-protector to CGO_CFLAGS

This fixes or addresses the following GitHub issues:

• Fixes Singularity binary cannot run images when compiled with FORTIFY_SOURCE=2 hardening #5429
• Fixes building 3.5.3 on alpine linux 3.12 failure #5346

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR and tested this PR locally with a make testall
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File

Attn: @singularity-maintainers

[dtrudg]

This would also fix #5346 I believe, due to enabling pie

[cclerget]

This would also fix #5346 I believe, due to enabling pie

Post updated, thanks !

[dtrudg]

This makes sense to me. Going to ask @tri-adam to cast his eyes over it too.

[tri-adam]

Nice work @cclerget, LGTM.