PowerPC: fix for gdb.base/eh_return.exp

PowerPC: fix for gdb.base/eh_return.exp

Disable the Traceback Table generation on PowerPC for this test.  The
Traceback Table consists of a series of bit fields to indicate things like
the Traceback Table version, language, and specific information about the
function.  The Traceback Table is generated following the end of the code
for every function by default.  The Traceback Table is defined in the
PowerPC ELF ABI and is intended to support debuggers and exception
handlers.  The Traceback Table is displayed in the disassembly of functions
by default and is part of the function length.  The table is typically
interpreted by the disassembler as data represented by .long xxx entries.

Generation of the Traceback Table is disabled in this test using the
PowerPC specific gcc compiler option -mtraceback=no, the xlc option
additional_flags-qtable=none and the clang optons
 -mllvm -xcoff-traceback-table=false.  Disabling the Traceback Table
generation in this test results in the gdb_test_multiple statement
correctly locating the address of the bclr instruction before the statement
"End of assembler dump." in the disassembly output.