cmd/internal/obj/riscv: add comments for Go registers

Change-Id: Id9aa6ba268eee67f2dc74096d4ec3bc0a80aefe2
Reviewed-on: https://go-review.googlesource.com/c/go/+/223563
Reviewed-by: Cherry Zhang <[email protected]>

Author: 4a6f656c

src/cmd/internal/obj/riscv/cpu.go

@@ -106,7 +106,7 @@ const (
 
 	// General registers reassigned to ABI names.
 	REG_ZERO = REG_X0
-	REG_RA   = REG_X1
+	REG_RA   = REG_X1 // aka REG_LR
 	REG_SP   = REG_X2
 	REG_GP   = REG_X3 // aka REG_SB
 	REG_TP   = REG_X4 // aka REG_G
@@ -125,7 +125,7 @@ const (
 	REG_A7   = REG_X17
 	REG_S2   = REG_X18
 	REG_S3   = REG_X19
-	REG_S4   = REG_X20
+	REG_S4   = REG_X20 // aka REG_CTXT
 	REG_S5   = REG_X21
 	REG_S6   = REG_X22
 	REG_S7   = REG_X23
@@ -136,7 +136,7 @@ const (
 	REG_T3   = REG_X28
 	REG_T4   = REG_X29
 	REG_T5   = REG_X30
-	REG_T6   = REG_X31
+	REG_T6   = REG_X31 // aka REG_TMP
 
 	// Go runtime register names.
 	REG_G    = REG_TP // G pointer.