cmake: Avoid appearing to support targets with dynamic timers

The code conditionally compiles against building on systems with
dynami timers.  Until this support can be added, be sure to not define
`RUST_SUPPORTED` on these targets.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

Kconfig

@@ -7,8 +7,9 @@ menu "Rust Language Support"
 
 config RUST_SUPPORTED
 	bool
-	default y if (CPU_CORTEX_M || \
-		(RISCV && !RISCV_ISA_RV32E && !RISCV_ISA_RV128I))
+	default y if ((CPU_CORTEX_M || \
+				(RISCV && !RISCV_ISA_RV32E && !RISCV_ISA_RV128I)) && \
+				!TIMER_READS_ITS_FREQUENCY_AT_RUNTIME)
 	help
 	  Selected for platforms that have support for Rust.