string-util: rework memory_erase() to not use GCC optimize attribute (#3812)

"#pragma GCC optimize" is merely a convenience to decorate multiple
functions with attribute optimize. And the manual has this to say about
this attribute:

  This attribute should be used for debugging purposes only. It
  is not suitable in production code.

Some versions of GCC also seem to have a problem with this pragma in
combination with LTO, resulting in ICEs.

So use a different approach (indirect the memset call via a volatile
function pointer) as implemented in openssl's crypto/mem_clr.c.

Closes: #3811

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