src: `enable_target_feature` should add features, not overwrite

`llvm_features` being empty is the default state, and implies the
presence of certain features.

Previously if any target features were explicitly enabled by the
`enable_target_feature` attribute, they were added comma separated
to `llvm_features`.

For example: `lzcnt,popcnt,...,sse4.2,sse`

This was causing LLVM to try to target a CPU that *ONLY* has the
explicitly enabled features.  This now will prefix explicitly enabled
features with a `+`, and preserve the existing `llvm_features` string
by appending to it if it is set.

0 files changed, 0 insertions, 0 deletions