riscv: refine vector context layout on stack (#1329)

Vector general register layout:

Before:
+--------------+  <-- High Address
|      v7      |
+--------------+
|      v6      |
+--------------+
|      ...     |
+--------------+
|      v0      |
+--------------+  <-- v0 - v7
|      v15     |
+--------------+
|      v14     |
+--------------+
|      ...     |
+--------------+
|      v8      |
+--------------+  <-- v8 - v15
|      ...     |
+--------------+
|      v24     |
+--------------+  <-- Low address

After:
+--------------+  <-- High Address
|      v31     |
+--------------+
|      v30     |
+--------------+
|      ...     |
+--------------+
|      v1      |
+--------------+
|      v0      |
+--------------+ <-- Low Address

Signed-off-by: wangfei_chen <[email protected]>
Co-authored-by: wangfei_chen <[email protected]>
Co-authored-by: Gaurav-Aggarwal-AWS <[email protected]>

Author: 3 people

portable/GCC/RISC-V/portContext.h

@@ -214,13 +214,13 @@ neg  t0, t0
 
 /* Store the vector registers in group of 8. */
 add     sp, sp, t0
-vs8r.v  v0, (sp)    /* Store v0-v7. */
-add     sp, sp, t0
-vs8r.v  v8, (sp)    /* Store v8-v15. */
+vs8r.v  v24, (sp)   /* Store v24-v31. */
 add     sp, sp, t0
 vs8r.v  v16, (sp)   /* Store v16-v23. */
 add     sp, sp, t0
-vs8r.v  v24, (sp)   /* Store v24-v31. */
+vs8r.v  v8, (sp)    /* Store v8-v15. */
+add     sp, sp, t0
+vs8r.v  v0, (sp)    /* Store v0-v7. */
 
 /* Store the VPU CSRs. */
 addi    sp, sp, -( 4 * portWORD_SIZE )
@@ -256,13 +256,13 @@ csrr t0, vlenb /* t0 = vlenb. vlenb is the length of each vector register in byt
 slli t0, t0, 3 /* t0 = vlenb * 8. t0 now contains the space required to store 8 vector registers. */
 
 /* Restore the vector registers. */
-vl8r.v  v24, (sp)
+vl8r.v  v0, (sp)    /* Restore v0-v7. */
 add     sp, sp, t0
-vl8r.v  v16, (sp)
+vl8r.v  v8, (sp)    /* Restore v8-v15. */
 add     sp, sp, t0
-vl8r.v  v8, (sp)
+vl8r.v  v16, (sp)   /* Restore v16-v23. */
 add     sp, sp, t0
-vl8r.v  v0, (sp)
+vl8r.v  v24, (sp)   /* Restore v23-v31. */
 add     sp, sp, t0
 
 /* Re-reserve the space for mstatus and epc. */