Fix mips build with GCC 9.

This patch fixes the build for MIPS (o32) with GCC 9 by stopping MIPS
__longjmp from using strong_alias, instead defining the alias
manually, so that the intended effect of not copying the nomips16
attribute is achieved, as explained in the included comment.

Tested with build-many-glibcs.py compilers build for mips64-linux-gnu
(which includes glibc builds for all three ABIs).

	* sysdeps/mips/__longjmp.c (__longjmp): Define alias manually with
	alias attribute, not with strong_alias.