Add --enable-static-pie variants to x86_64, x32 and i686

Since the default GCC and binutils versions used by build-many-glibcs.py,
which are GCC 7 branch and binutils 2.29 branch, support static PIE on
x86_64, x32 and i686, this patch adds --enable-static-pie glibc variants
to x86_64, x32 and i686 to get some coverage for static PIE.

Tested with build-many-glibcs.py.

	* scripts/build-many-glibcs.py (Context.add_all_configs): Add
	--enable-static-pie variants to x86_64, x32 and i686.