target/riscv: vector CSRs are optional

This is a fix to a mistake made in
ea7e174.

The newly introduced `gdb_regno_exist()` function was missing a part
regarding vector CSRs.
Link: riscv-collab@ea7e174#diff-b4aa16f9e42cb8f0934baa7c8e0ec9c70a369bef98b99b26ae2e896c8aa95d6aL6163-L6171

Change-Id: I0361ea4dce8df5be748e2c6e7e6838029d3a7120
Signed-off-by: Evgeniy Naydanov <[email protected]>

Author: en-sc

src/target/riscv/riscv_reg.c

@@ -397,6 +397,14 @@ static bool gdb_regno_exist(const struct target *target, uint32_t regno)
 		case CSR_FRM:
 		case CSR_FCSR:
 			return riscv_supports_extension(target, 'F');
+		case CSR_VSTART:
+		case CSR_VXSAT:
+		case CSR_VXRM:
+		case CSR_VL:
+		case CSR_VCSR:
+		case CSR_VTYPE:
+		case CSR_VLENB:
+			return vlenb_exists(target);
 		case CSR_SCOUNTEREN:
 		case CSR_SSTATUS:
 		case CSR_STVEC: