libstdc++: Declare const global variables inline

libstdc++: Declare const global variables inline

The changes inside the regex_constants and execution namespaces seem to
be (the only) unimplemented parts of P0607R0 "Inline Variable for the
Standard Library"; the rest of the changes are to implementation details.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (_detail::__platform_wait_alignment):
	Declare inline.  Remove redundant static specifier.
	(__detail::__atomic_spin_count_relax): Declare inline.
	(__detail::__atomic_spin_count): Likewise.
	* include/bits/regex_automaton.h (__detail::_S_invalid_state_id):
	Declare inline for C++17.  Declare constexpr.  Remove
	redundant const and static specifiers.
	* include/bits/regex_error.h (regex_constants::error_collate):
	Declare inline for C++17 as per P0607R0.
	(regex_constants::error_ctype): Likewise.
	(regex_constants::error_escape): Likewise.
	(regex_constants::error_backref): Likewise.
	(regex_constants::error_brack): Likewise.
	(regex_constants::error_paren): Likewise.
	(regex_constants::error_brace): Likewise.
	(regex_constants::error_badbrace): Likewise.
	(regex_constants::error_range): Likewise.
	(regex_constants::error_space): Likewise.
	(regex_constants::error_badrepeat): Likewise.
	(regex_constants::error_complexity): Likewise.
	(regex_constants::error_stack): Likewise.
	* include/ext/concurrence.h (__gnu_cxx::__default_lock_policy):
	Likewise.  Remove redundant static specifier.
	* include/pstl/execution_defs.h (execution::seq): Declare inline
	for C++17 as per P0607R0.
	(execution::par): Likewise.
	(execution::par_unseq): Likewise.
	(execution::unseq): Likewise.

(cherry picked from commit e3b10249119fb4258fb83d21d805a04f30b63152)