Fix #19052; [backport:1.6.0] (#19053)

* Fix #19052; [backport:1.6.0]

Adds a compile flag to avoid a getrandom syscall, fixing #19052.

This is neccesary when the getrandom syscall is missing, as noted in #19052, particularly in kernel versions < 3.17 when getrandom was introduced. Specifically relevant is this is missing from kernel 3.10, which is the supported kernel throughout RHEL 7 and CentOS 7, which is widely used at many organizations. Without this, versions of nim that include sysrand (i.e. versions >= 1.6.0) will not compile without modification, however with this change a compile flag may be used to fall back using /dev/urandom as done with any unknown Posix OS (preferred here as a fallback since it already supplies a cryptographically secure PRNG and existing code deals with entropy pool init, etc).

The change is placed behind a compile flag, as discussed in github ticket #19052 (summed up here):
* First, I can't seem to catch that a importc such as SYS_getrandom is declared without using it (the declared proc returns true, but compiler throws an undeclared identifier flag when referencing it).
* Second, it seemed preferable to be behaviorally explicit vs implicit when considering this is intended to be a cryptographically secure PRNG.
* Third, if I intend to compile on a kernel >= 3.17 while running the binary on at least one system < 3.17, I'll want to be able to target this without relying on a compile time determination if the getrandom syscall is available.

* Documenting compile flag for -d:nimNoGetRandom and adding changelog entry
Related to #19052 and comments in PR #19053. Also created a new changelog file since none currently exists.

Co-authored-by: Timothy Alexander <talexander@midwestlabs.com>

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