[patch] libstdc++/64847 add autoconf checks for pthread_rwlock_t

[Jonathan Wakely]

I assumed that Pthreads was enough to ensure pthread_rwlock_t but
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64847 shows that isn't
true for HPUX (seems it was optional prior to POSIX 1003.1-2001).

This adds an autoconf check to decide whether to use pthread_rwlock_t
or the fallback implementation in terms of std::condition_variable and
std::mutex.

This also includes some fixes from Torvald so that we loop and retry
if libc returns EAGAIN, to handle a difference in semantics between
POSIX and C++14.

And as an optimization I've made the _M_rwlock member use the
PTHREAD_RWLOCK_INITIALIZER macro if available.

Tested x86_64-linux, ppc64le-linux and x86_64-dragonfly.

I plan to commit this to trunk tomorrow.

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