setjmp.h: Fix C++ build and avoid duplicate throw declaration

setjmp.h: Fix C++ build and avoid duplicate throw declaration

Commit 21cbb6fe ("unistd.h: put getppid under XOPEN2K8") introduced a
new __THROWNL specification for non-leaf throws. It also made use of
these in the setjmp.h header. The functions that use this specification
(longjmp and siglongjmp) have their extern __libc__* equivalent
definition prototype specified using __typeof__ for the internal
function signatures. For C++ this copies the throw() specifier, since
this is part of the type for C++. The attribute in C is not.

This commit explicitly types out the signature for the two functions to
be compatible between the C++ and C worlds.

An alternative would be to keep the __typeof__ declaration and use
the copy attribute. Then the __THROWNL part could be thrown out since
the C attribute would be copied and the C++ exception specifier would
be part of the signature from __type__. However, since the copy
attribute is not supported for all compilers supported by uclibc-ng
this is not viable at this time.