The enhancement to detect mismatched attributes between function
aliases and their targets triggers (expected) warnings in GCC
builds due to aliases being declared with fewer attributes than
their targets.

Using attribute copy as recommended to copy the attributes from
the target to the alias triggers another warning, this time due
to applying attribute leaf to static functions (the attribute
only applies to extern functions).  This is due to an oversight
in both the handler for attribute copy and in
the -Wmissing-attributes warning.

In addition, the copy attribute handler doesn't account for C11
_Noreturn and C++ throw() specifications, both of which set
the corresponding tree bits but don't attach the synonymous
attribute to it.  This also leads to warnings in GCC builds
(in libgfortran).

The attached patch corrects all of these problems: the attribute
copy handler to avoid copying attribute leaf to declarations of
static functions, and to set the noreturn and nonthrow bits, and
the missing attribute warning to avoid triggering for static
weakref aliases whose targets are decorated wiwth attribute leaf.

With this patch, GCC should build with no -Wmissing-attributes
warnings.

Tested on x86_64-linux.

Martin