Big-endian targets: Fix implptrpiece.exp

The test case implptrpiece.exp accesses the second byte of the short
integer number 1 and expects it to be zero.  This is valid for
little-endian targets, but fails on big-endian targets.

This is fixed by distinguishing the expected value by endianness.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/implptrpiece.exp: Fix check for big-endian targets.