Update the X86 assembler for .intel_syntax to produce an error for invalid
scale factors in memory addresses. As it does for .att_syntax.

It was producing:
Assertion failed: (((Scale == 1 || Scale == 2 || Scale == 4 || Scale == 8)) && "Invalid scale!"), function CreateMem, file /Volumes/SandBox/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp, line 1133.

rdar://14967214
Add a "step-avoid-libraries" setting to complement the "step-avoid-regexp" setting.