stm32f3: fix nonexistent cfg tests

The rcc code was taken from stm32-rs which uses 'x' features, but
embassy uses features with full chip names.

Add these 'x' wildcards as cfgs and use them in rcc.
They will be useful for USB too.

embassy-stm32/build.rs

@@ -125,6 +125,10 @@ fn main() {
         println!("cargo:rustc-cfg={}", &chip_name[..chip_name.len() - 2]);
     }
 
+    if chip_name.starts_with("stm32f3") {
+        println!("cargo:rustc-cfg={}x{}", &chip_name[..9], &chip_name[10..11]);
+    }
+
     // ========
     // Handle time-driver-XXXX features.
 

embassy-stm32/src/rcc/f3.rs

@@ -217,8 +217,8 @@ fn calc_pll(config: &Config, Hertz(sysclk): Hertz) -> (Hertz, PllConfig) {
             cfg_if::cfg_if! {
                 // For some chips PREDIV is always two, and cannot be changed
                 if #[cfg(any(
-                        feature="stm32f302xd", feature="stm32f302xe", feature="stm32f303xd",
-                        feature="stm32f303xe", feature="stm32f398xe"
+                        stm32f302xd, stm32f302xe, stm32f303xd,
+                        stm32f303xe, stm32f398xe
                     ))] {
                     let (multiplier, divisor) = get_mul_div(sysclk, HSI);
                     (