struct _Unwind_Exception alignment should not depend on compiler flags

__attribute__((__aligned__)) selects an alignment that depends on
the micro-architecture selected by GCC flags.  Enabling vector
extensions may increase the allignment.  This is a problem when
building glibc as a collection of ELF multilibs with different
GCC flags because ld.so and libc.so/libpthread.so/&c may end up
with a different layout of struct pthread because of the
changing offset of its struct _Unwind_Exception field.

Tested-By: Matheus Castanho <msc@linux.ibm.com>