gdb.trace: Fix unavailable-dwarf-piece.exp on big endian targets

gdb.trace: Fix unavailable-dwarf-piece.exp on big endian targets

The test constructs fake DWARF info for a C structure involving bitfields.
DWARF bitfields are always counted from LSB, while the order in which
bitfields are allocated in a C struct depends on the target endianness -
thus the generated DWARF marks different bitfields as unavailable when
target is big endian.  Accordingly, we need different expected outputs.

Tested on s390 and s390x, no regression on x86_64.

gdb/testsuite/ChangeLog:

	* gdb.trace/unavailable-dwarf-piece.exp: Fix bitfield handling on big
	endian targets.