Remove codegen dependencies

Not doing this leads to building two copies of e.g. num_cpus in the
sysroot and _llvm deps, leading to conflicts between the two when
compiling librustc_codegen_llvm. It's not entirely clear why this is the
case after the changes in this PR but likely has something to do with a
subtle difference in ordering or similar.

Cargo.lock

@@ -3129,11 +3129,7 @@ dependencies = [
 name = "rustc_codegen_llvm"
 version = "0.0.0"
 dependencies = [
- "cc",
- "memmap",
- "num_cpus",
  "rustc_llvm",
- "tempfile",
 ]
 
 [[package]]

src/librustc_codegen_llvm/Cargo.toml

@@ -11,11 +11,7 @@ crate-type = ["dylib"]
 test = false
 
 [dependencies]
-cc = "1.0.1" # Used to locate MSVC
-num_cpus = "1.0"
-tempfile = "3.0"
 rustc_llvm = { path = "../librustc_llvm" }
-memmap = "0.6"
 
 [features]
 # This is used to convince Cargo to separately cache builds of `rustc_codegen_llvm`