RISC-V: Don't use a global include guard for uapi/asm/syscalls.h

This file is expected to be included multiple times in the same file in
order to allow the __SYSCALL macro to generate system call tables.  With
a global include guard we end up missing __NR_riscv_flush_icache in the
syscall table, which results in icache flushes that escape the vDSO call
to not actually do anything.

The fix is to move to per-#define include guards, which allows the
system call tables to actually be populated.  Thanks to Macrus Comstedt
for finding and fixing the bug!

Cc: Marcus Comstedt <marcus@mc.pp.se>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/unistd.h

@@ -11,6 +11,11 @@
  *   GNU General Public License for more details.
  */
 
+/*
+ * There is explicitly no include guard here because this file is expected to
+ * be included multiple times.  See uapi/asm/syscalls.h for more info.
+ */
+
 #define __ARCH_WANT_SYS_CLONE
 #include <uapi/asm/unistd.h>
 #include <uapi/asm/syscalls.h>