Fix F register count to 32 for RV32E with F extension

According to RV32E Base Integer Instruction Set, Version 1.9 [1]:

RV32E can be combined with all current standard extensions. Defining the
F, D, and Q extensions as having a 16-entry floating point register file
when combined with RV32E was considered but decided against.

When RV32E is used with floating-point extensions, the floating-point
register file should have 32 entries instead of N_RV_REGS (16).

Adjust array size accordingly.

[1] https://five-embeddev.com/riscv-user-isa-manual/Priv-v1.12/rv32e.html#rv32e

Author: eleanorLYJ

src/riscv_private.h

@@ -131,7 +131,7 @@ struct riscv_internal {
 
 #if RV32_HAS(EXT_F)
     /* float registers */
-    riscv_float_t F[N_RV_REGS];
+    riscv_float_t F[32];
     uint32_t csr_fcsr;
 #endif