gdb: Powerpc fix gdb.multi/multi-arch-exec.exp test

gdb: Powerpc fix gdb.multi/multi-arch-exec.exp test

The expect file has a procedure append_arch_options which sets march based
the istarget.  The current if / else statement does not check for
powerpc64.  The else statement is hit which sets march to -m32.  This
results in compilation errors on 64-bit PowerPC.

This patch adds an if statement to check for powerpc64 and if true sets mach
to -m64.

The patch was tested on a Power 10 system.  No compile errors were generated.
The test completes with 1 expected pass and no failures.