refactor: autodetect macro variant

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

embassy-macros/Cargo.toml

@@ -21,9 +21,5 @@ proc-macro2 = "1.0.29"
 proc-macro = true
 
 [features]
-std = []
-wasm = []
-riscv = []
-
 # Enabling this cause interrupt::take! to require embassy-executor
 rtos-trace-interrupt = []