[patch, testsuite] restrict gdb.arch/ppc64-symtab-cordic.exp to ppc64 targets

[Sandra Loosemore]

I've found that it doesn't do anything useful to try loading a ppc64 .ko 
file when testing GDB on ARM, Nios II, etc.  I copied the target test 
logic from the other gdb.arch/ppc64-*.exp testcase -- is this OK to 
check in?

-Sandra

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