Reduce compiler ability to optimise to statisfy gcc 9.5

[torben-hansen]

Issues:

P118709392

Description of changes:

This PR is an attempt to fix a transient issue with gcc 9.5 compilation. This is a test workflow, so no hot-path performance regression concerns from me.

gcc 9.5 has a bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189 that has not been backported. Essentially, the compiler will falsely reason that it's comparing a string and optimise out everything behind a null-character.

In the test fixture SSLVersionTest.DefaultTicketKeyInitialization it appears the gcc 9.5 compiler only compares the first byte and short-circuits there. What was observed:

4a6274:	394243e0 	ldrb	w0, [sp, #144] <-- first byte of ticket_key
  4a6278:	b90043ff 	str	wzr, [sp, #64] <-- zeroise stack (kzerokey presumably)
  4a627c:	b90053e0 	str	w0, [sp, #80] <-- Put w0 back on the stack(?)
  4a6280:	34000240 	cbz	w0, 4a62c8 <-- zero-test register w0, branch to failure path (adr 4a62c8) if true <_ZN4bssl12_GLOBAL__N_150SSLVersionTest_DefaultTicketKeyInitialization_Test8TestBodyEv+0x128>

Dropping static const removes optimisation options for the compiler. In my tests, gcc 9.5 now emits a direct memcmp instead of attempting any optimisations:

  4a89a4:	aa1603e0 	mov	x0, x22
  4a89a8:	d2800602 	mov	x2, #0x30                  	// #48
  4a89ac:	b90043ff 	str	wzr, [sp, #64]
  4a89b0:	910243e1 	add	x1, sp, #0x90
  4a89b4:	97fd915f 	bl	40cf30 <memcmp@plt>

Testing:

Tested on machines with gcc 9.5 runtime. Before failed, after this patch, no failure (even when forcing first byte to 0).

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7f76c04) 76.85% compared to head (47a9a78) 76.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1442   +/-   ##
=======================================
  Coverage   76.85%   76.86%           
=======================================
  Files         425      425           
  Lines       71533    71533           
=======================================
+ Hits        54980    54985    +5     
+ Misses      16553    16548    -5     

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