Sync scratch_buffer with gnulib

This patch syncs the scratch_buffer grom gnulib commit 3866ef6 with
GLIBC code.

Checked on x86_64-linux-gnu and on a build using build-many-glibcs.py
for all major architectures.

	* include/scratch_buffer.h (scratch_buffer): Use a C99 align method
	instead of GCC extension.
	* malloc/scratch_buffer_grow.c [!_LIBC]: Include libc-config.h.
	* malloc/scratch_buffer_grow_preserve.c [!_LIBC]: Likewise.
	* malloc/scratch_buffer_set_array_size.c [!_LIBC]: Likewise.