refactor: autodetect macro variant

Export all main macro per target architecture from embassy-macros,
and select the appropriate macro in embassy-executor.

embassy-executor/Cargo.toml

@@ -29,9 +29,8 @@ flavors = [
 
 [features]
 default = []
-std = ["embassy-macros/std", "critical-section/std"]
-wasm = ["dep:wasm-bindgen", "dep:js-sys", "embassy-macros/wasm"]
-riscv = ["embassy-macros/riscv"]
+std = ["critical-section/std"]
+wasm = ["dep:wasm-bindgen", "dep:js-sys"]
 
 # Enable nightly-only features
 nightly = []