Handle tilegx* machine names.

This patch makes sysdeps/tile/preconfigure handle tilegx* machine
names instead of just plain tilegx.  That matches GCC, and in
particular allows a big-endian toolchain to use the tilegxbe-linux-gnu
name when configuring both GCC and glibc.

Tested with compilation for tilegxbe-linux-gnu, both 32-bit and 64-bit
(building a subsequent GCC against that glibc still falls over because
of both 32-bit and 64-bit libraries going in the lib directory, as
noted at
<https://sourceware.org/ml/libc-alpha/2016-11/msg00129.html>).

	* sysdeps/tile/preconfigure: Accept tilegx* instead of tilegx.