Restore getrandom(2) path for Linux with glibc 2.36 or later

This is a follow-up to commit b120f5e38d9c (avoid fork-unsafe arc4random
implementations, 2018-09-04).

Avoid defining a no-op fill_random_bytes_syscall() if arc4random_buf(3)
exists, but we are unsure if it is fork-safe. Check for other options
instead. IOW, see if getrandom(2) is available.

glibc 2.36, released in 2022, started to provide arc4random_buf(3) on
Linux. This causes fill_random_bytes_syscall() to use neither of them
and makes Random.urandom solely rely on getentropy(3) via
fill_random_bytes_urandom().

While the glibc implementation is safe, I did not add it to the list
because using getrandom(2) directly is preferable on Linux.

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