[PowerPC][AIX] Don't hardcode python invoke command line

We shouldn't assume python exists, we should let lit
to decide whether it is python or python3 and expand the path.

Author: Jinsong Ji

llvm/test/CodeGen/PowerPC/aix-overflow-toc.py

@@ -1,6 +1,6 @@
 # UNSUPPORTED: expensive_checks, debug
 
-# RUN: python %s > %t.ll
+# RUN: %python %s > %t.ll
 # RUN: llc -mtriple powerpc-ibm-aix-xcoff -code-model=small -mcpu=pwr4 -mattr=-altivec -O0 < %t.ll | \
 # RUN:   FileCheck --check-prefix=ASM32 %s