It's not difficult for multiple threads to drain the entropy available
to the RDSEED instruction, at which point we throw an exception. This
change will try to use RDRAND after RDSEED fails repeatedly, and only
throw if RDRAND also fails repeatedly. This doesn't guarantee a random
value can always be read, but reduces the likelihood of failure when
using the RDSEED instruction.
PR libstdc++/94087
* src/c++11/random.cc (__x86_rdseed): Allow fallback function to be
passed in.
(__x86_rdseed_rdrand): New function that uses rdseed with rdrand
fallback.
(random_device::_M_init): Use __x86_rdseed_rdrand when both
instructions are available.
* testsuite/26_numerics/random/random_device/94087.cc: New test.
(cherry picked from commit a2d196e75cef95c2b70734ad02e94f9da0e769fe)
