RISC-V: Stop relying on GCC's register allocator's hueristics

GCC allows users to hint to the register allocation that a variable should be
placed in a register by using a syntax along the lines of

    function(...) {
        register long in_REG __asm__("REG");
    }

We've abused this a bit throughout the RISC-V port to access fixed registers
directly as C variables.  In practice it's never going to blow up because GCC
isn't going to allocate these registers, but it's not a well defined syntax so
we really shouldn't be relying upon this.  Luckily there is a very similar but
well defined syntax that allows us to still access these registers directly as
C variables, which is to simply declare the register variables globally.  For
fixed variables this doesn't change the ABI.

LLVM disallows this ambiguous syntax, so this isn't just strictly a formatting
change.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/process.c

@@ -22,6 +22,8 @@
 #include <asm/switch_to.h>
 #include <asm/thread_info.h>
 
+unsigned long gp_in_global __asm__("gp");
+
 extern asmlinkage void ret_from_fork(void);
 extern asmlinkage void ret_from_kernel_thread(void);
 
@@ -107,9 +109,8 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp,
 	/* p->thread holds context to be restored by __switch_to() */
 	if (unlikely(p->flags & PF_KTHREAD)) {
 		/* Kernel thread */
-		const register unsigned long gp __asm__ ("gp");
 		memset(childregs, 0, sizeof(struct pt_regs));
-		childregs->gp = gp;
+		childregs->gp = gp_in_global;
 		/* Supervisor/Machine, irqs on: */
 		childregs->status = SR_PP | SR_PIE;