libstdc++: Mark non-exported function always_inline [PR105671]

This new function was added for gcc 11.1 but is not exported from the
shared library. Depending on inlining decisions, its callers might get
inlined but an external definition be needed for this function. That
then fails to link.

Since we can't add the export to the gcc-11 release branch now, mark it
always_inline. We can consider exporting it for gcc-13 if/when we bump
the shared library version (and maybe also for gcc-12 which is currently
at the same version as trunk). For now, the attribute will solve the
problem on all affected branches. The function is small enough that
force-inlining it shouldn't cause problems.

libstdc++-v3/ChangeLog:

	PR libstdc++/105671
	* include/std/sstream (basic_stringbuf::_M_high_mark): Add
	always_inline attribute.

libstdc++-v3/include/std/sstream

@@ -425,6 +425,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // This might not be the same character as _M_string.end() because
       // basic_stringbuf::overflow might have written to unused capacity
       // in _M_string without updating its length.
+      __attribute__((__always_inline__))
       char_type*
       _M_high_mark() const _GLIBCXX_NOEXCEPT
       {