Problem reported by Florian Weimer [1] and solution suggested by
Andreas Schwab [2]. It also set the same buffer size independent
of architecture max_align_t size.
Checked on x86_64-linux-gnu and i686-linux-gnu.
* lib/malloc/scratch_buffer.h (struct scratch_buffer):
Use an union instead of a max_align_t array for __space,
so that __space is the same size on all platforms.
* malloc/scratch_buffer_grow_preserve.c
(__libc_scratch_buffer_grow_preserve): Likewise.
[1] https://sourceware.org/ml/libc-alpha/2017-09/msg00693.html
[2] https://sourceware.org/ml/libc-alpha/2017-09/msg00695.html
