[rs6000] Create a powerpc-power10.exp test

Inspired by the existing powerpc-power9.exp test, this is a
new test to cover the power10 instruction disassembly.

gdb/testsuite/ChangeLog:
2021-04-12  Will Schmidt  <will_schmidt@vnet.ibm.com>

	* gdb.arch/powerpc-power10.s: New test for instructions.
	* gdb.arch/powerpc-power10.exp: Harness to run the test.