Fix MicroBlaze bits/setjmp.h for C++.

Fix MicroBlaze bits/setjmp.h for C++.

For MicroBlaze, setjmp/check-installed-headers-cxx fails with:

../setjmp/setjmp.h:34:8: error: '__jmp_buf_tag' has a field '__jmp_buf_tag::__jmpbuf' whose type depends on the type '<unnamed struct>' which has no linkage [-Werror=subobject-linkage]

This patch fixes this in the same way as for some other architectures:
the struct used for the internal __jmp_buf type is given the tag
__jmp_buf_internal_tag.

Tested (compilation tests) with build-many-glibcs.py.

	* sysdeps/microblaze/bits/setjmp.h (__jmp_buf): Give struct tag
	__jmp_buf_internal_tag.