[X86] Move the implicit enabling of sse2 for 64-bit mode from...

[X86] Move the implicit enabling of sse2 for 64-bit mode from X86Subtarget::initSubtargetFeatures to X86_MC::ParseX86Triple.

ParseX86Triple already checks for 64-bit mode and produces a
static string. We can just add +sse2 to the end of that static
string. This avoids a potential reallocation when appending it
to the std::string at runtime.

This is a slight change to the behavior of tools that only use
MC layer which weren't implicitly enabling sse2 before, but will
now. I don't think we check for sse2 explicitly in any MC layer
components so this shouldn't matter in practice. And if it did
matter the new behavior is more correct.