simplify docs further

Author: ZuseZ4

src/doc/rustc-dev-guide/src/offload/usage.md

@@ -68,10 +68,9 @@ pub extern "gpu-kernel" fn kernel_1(x: *mut [f64; 256]) {
 ## Compile instructions
 It is important to use a clang compiler build on the same llvm as rustc. Just calling clang without the full path will likely use your system clang, which probably will be incompatible. So either substitute clang/lld invocations below with absolute path, or set your `PATH` accordingly.
 
-First we generate the host (cpu) code. The first build is just to compile libc, take note of the hashed path. Then we call rustc directly to build our host code, while providing the libc artifact to rustc.
+First we generate the host (cpu) code.
 ```
-cargo +offload build -r -v
-rustc +offload --edition 2024 src/lib.rs -g --crate-type cdylib -C opt-level=3 -C panic=abort -C lto=fat -L dependency=/absolute_path_to/target/release/deps --extern libc=/absolute_path_to/target/release/deps/liblibc-<HASH>.rlib --emit=llvm-bc,llvm-ir  -Zoffload=Enable -Zunstable-options
+RUSTFLAGS=" --emit=llvm-bc,llvm-ir -Zoffload=Enable -Zunstable-options" cargo +offload build -r
 ```
 
 Now we generate the device code. Replace the target-cpu with the right code for your gpu.
@@ -99,6 +98,7 @@ The first element is zero 0.000000
 The first element is NOT zero 21.000000
 The second element is  0.000000
 ```
+If this fails, check whether you've loaded the right drivers or runtime. E.g. for an AMD GPU, you want rocm (or more specific HSA) available.
 
 To receive more information about the memory transfer, you can enable info printing with
 ```