shells/bash: fix build on powerpc64le/CURRENT
Clang crashes with the default -mcpu, but builds with -O0:
Assertion failed: (Subtarget->hasP9Vector() && "XSCMPUQP requires Power9 Vector"), function SelectCC, file /usr/src/contrib/llvm-project/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp, line 4271.