tools: riscv: Add header file vdso/processor.h

Borrow the cpu_relax() definitions from kernel's
arch/riscv/include/asm/vdso/processor.h to tools/ for riscv.
Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

tools/arch/riscv/include/asm/vdso/processor.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_VDSO_PROCESSOR_H
+#define __ASM_VDSO_PROCESSOR_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm-generic/barrier.h>
+
+static inline void cpu_relax(void)
+{
+#ifdef __riscv_muldiv
+	int dummy;
+	/* In lieu of a halt instruction, induce a long-latency stall. */
+	__asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
+#endif
+
+#ifdef CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE
+	/*
+	 * Reduce instruction retirement.
+	 * This assumes the PC changes.
+	 */
+	__asm__ __volatile__ ("pause");
+#else
+	/* Encoding of the pause instruction */
+	__asm__ __volatile__ (".4byte 0x100000F");
+#endif
+	barrier();
+}
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* __ASM_VDSO_PROCESSOR_H */