PR libstdc++/85494 use rdseed and rand_s in std::random_device

PR libstdc++/85494 use rdseed and rand_s in std::random_device

Add support for additional sources of randomness to std::random_device,
to allow using RDSEED for Intel CPUs and rand_s for Windows. When
supported these can be selected using the tokens "rdseed" and "rand_s".
For *-w64-mingw32 targets the "default" token will now use rand_s, and
for other i?86-*-* and x86_64-*-* targets it will try to use "rdseed"
first, then "rdrand", and finally "/dev/urandom".

To simplify the declaration of std::random_device in <bits/random.h> the
constructors now unconditionally call _M_init instead of _M_init_pretr1,
and the function call operator now unconditionally calls _M_getval. The
library code now decides whether _M_init and _M_getval should use a real
source of randomness or the mt19937 engine.

Existing code compiled against old libstdc++ headers will still call
_M_init_pretr1 and _M_getval_pretr1, but those functions now forward to
_M_init and _M_getval if a real source of randomness is available. This
means existing code compiled for mingw-w64 will start to use rand_s just
by linking to a new libstdc++.dll.

	* acinclude.m4 (GLIBCXX_CHECK_X86_RDSEED): Define macro to check if
	the assembler supports rdseed.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_CHECK_X86_RDSEED.
	* config/os/mingw32-w64/os_defines.h (_GLIBCXX_USE_CRT_RAND_S): Define.
	* doc/html/*: Regenerate.
	* doc/xml/manual/status_cxx2011.xml: Document new tokens.
	* include/bits/random.h (random_device::random_device()): Always call
	_M_init rather than _M_init_pretr1.
	(random_device::random_device(const string&)): Likewise.
	(random_device::operator()()): Always call _M_getval().
	(random_device::_M_file): Replace first member of union with an
	anonymous struct, with _M_file as its first member.
	* src/c++11/random.cc [_GLIBCXX_X86_RDRAND] (USE_RDRAND): Define.
	[_GLIBCXX_X86_RDSEED] (USE_RDSEED): Define.
	(USE_MT19937): Define if none of the above are defined.
	(USE_POSIX_FILE_IO): Define.
	(_M_strtoul): Remove.
	[USE_RDSEED] (__x86_rdseed): Define new function.
	[_GLIBCXX_USE_CRT_RAND_S] (__winxp_rand_s): Define new function.
	(random_device::_M_init(const string&)): Initialize new union members.
	Add support for "rdseed" and "rand_s" tokens. Decide what the
	"default" token does according to which USE_* macros are defined.
	[USE_POSIX_FILE_IO]: Store a file descriptor.
	[USE_MT19937]: Forward to _M_init_pretr1 instead.
	(random_device::_M_init_pretr1(const string&)) [USE_MT19937]: Inline
	code from _M_strtoul.
	[!USE_MT19937]: Call _M_init, transforming the old default token or
	numeric tokens to "default".
	(random_device::_M_fini()) [USE_POSIX_FILE_IO]: Use close not fclose.
	(random_device::_M_getval()): Use new union members to obtain a
	random number from the stored function pointer or file descriptor.
	[USE_MT19937]: Obtain a value from the mt19937 engine.
	(random_device::_M_getval_pretr1()): Call _M_getval().
	(random_device::_M_getentropy()) [USE_POSIX_FILE_IO]: Use _M_fd
	instead of fileno.
	[!USE_MT19937] (mersenne_twister): Do not instantiate when not needed.
	* testsuite/26_numerics/random/random_device/85494.cc: New test.

From-SVN: r271740