cmd/compile,cmd/internal/obj/riscv: move g register on riscv64

The original riscv64 port used the thread pointer (TP aka X4) register for
the g pointer, however this register is also used when TLS support is
required, resulting in a conflict (for example, when a signal is received
we have no way of readily knowing if X4 contains a pointer to the TCB or
a pointer to a g).

In order to support cgo, free up the X4 register by moving g to X27.
This unfortunately means that the X4 register is unused in non-cgo mode,
however the alternative is to not support cgo on this platform.

Update #36641

Change-Id: Idcaf3e8ccbe42972a1b8943aeefde7149d9c960a
Reviewed-on: https://go-review.googlesource.com/c/go/+/263477
Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/RISCV64Ops.go

@@ -24,10 +24,11 @@ import (
 // L               = 64 bit int, used when the opcode starts with F
 
 const (
-	riscv64REG_G    = 4
+	riscv64REG_G    = 27
 	riscv64REG_CTXT = 20
 	riscv64REG_LR   = 1
 	riscv64REG_SP   = 2
+	riscv64REG_TP   = 4
 	riscv64REG_TMP  = 31
 	riscv64REG_ZERO = 0
 )
@@ -78,8 +79,8 @@ func init() {
 
 		// Add general purpose registers to gpMask.
 		switch r {
-		// ZERO, and TMP are not in any gp mask.
-		case riscv64REG_ZERO, riscv64REG_TMP:
+		// ZERO, TP and TMP are not in any gp mask.
+		case riscv64REG_ZERO, riscv64REG_TP, riscv64REG_TMP:
 		case riscv64REG_G:
 			gpgMask |= mask
 			gpspsbgMask |= mask