machine/riscv: Use __riscv_flen instead of __riscv_f

__riscv_f is an old symbol which isn't the standard name and isn't
available in all toolchains.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

newlib/libc/machine/riscv/sys/fenv.h

@@ -14,7 +14,7 @@
 
 #include <stddef.h>
 
-#if defined(__riscv_f) || defined(__riscv_zfinx)
+#if defined(__riscv_flen) || defined(__riscv_zfinx)
 
 /* Per "The RISC-V Instruction Set Manual: Volume I: User-Level ISA:
  * Version 2.1", Section 8.2, "Floating-Point Control and Status