[ExecutionEngine] Allow JIT tests to run on PowerPC.

These tests passed on my ppc64le test machine. If they survive testing by the
buildbots then we can leave them enabled, and this will allow us to land the
new ppc64 JITLink backend (https://reviews.llvm.org/D148192).

llvm/test/ExecutionEngine/lit.local.cfg

@@ -1,4 +1,4 @@
-if config.root.native_target in ['Sparc', 'PowerPC', 'SystemZ', 'Hexagon', 'RISCV']:
+if config.root.native_target in ['Sparc', 'SystemZ', 'Hexagon', 'RISCV']:
     config.unsupported = True
 
 # ExecutionEngine tests are not expected to pass in a cross-compilation setup.