[PATCH 0 of 4] Avoid dead store elimination in GCC 11+

[Vadim Fedorenko via nginx-devel]

GCC version 11 and newer use more aggressive way to eliminate dead stores
which ends up removing ngx_memzero() calls in several places. Such optimization
affects calculations of md5 and sha1 implemented internally in nginx. The
effect could be easily observed by adding a random data to buffer array in
md5_init() or sha1_init() functions. With this simple modifications the result
of the hash computation will be different each time even though the provided
data to hash is not changed. Changing the code to use current implementation
of ngx_explicit_memzero() doesn't help because of link-time optimizations
enabled in RHEL 9 and derivatives. Glibc 2.34 found in RHEL 9 provides
explicit_bzero() function which should be used to avoid such optimization.
ngx_explicit_memzero() is changed to use explicit_bzero() if possible.
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