timing: replace deprecated gettimeofday() with clock()

[lanodan]

Description

gettimeofday() got marked obsolete in POSIX.1-2008 and removed
in POSIX.1-2024.

Alternatives:

• clock() introduced in C89, part of POSIX.1-1995 (and meant for CPU time)
• timespec_get() introduced in C11 and part of POSIX.1-2024
• clock_gettime() introduced in POSIX.1-1995,
  absent in ISO C so requires POSIX feature macros

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line.
If the provided content is part of the present PR remove the # symbol.

• changelog not required because: internal change
• development PR this is against development branch
• TF-PSA-Crypto PR not required because: internal change
• framework PR not required: internal change
• 3.6 PR provided # | not required because: (waiting for first feedback, applies cleanly)
• tests not required because: not an API change

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