x86/syscalls: Clear 'offset' and 'prefix' in case they are set in env

If the environment variable 'prefix' is set on the build host, it is
wrongly used as syscall macro prefixes.

  $ export prefix=/usr
  $ make -s defconfig all
  In file included from ./arch/x86/include/asm/unistd.h:20,
                   from <stdin>:2:
  ./arch/x86/include/generated/uapi/asm/unistd_64.h:4:9: warning: missing whitespace after the macro name
      4 | #define __NR_/usrread 0
        |         ^~~~~

arch/x86/entry/syscalls/Makefile should clear 'offset' and 'prefix'.

Fixes: 3cba325b ("x86/syscalls: Switch to generic syscallhdr.sh")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210525115420.679416-1-masahiroy@kernel.org

arch/x86/entry/syscalls/Makefile

@@ -11,6 +11,8 @@ syscall64 := $(src)/syscall_64.tbl
 
 syshdr := $(srctree)/scripts/syscallhdr.sh
 systbl := $(srctree)/scripts/syscalltbl.sh
+offset :=
+prefix :=
 
 quiet_cmd_syshdr = SYSHDR  $@
       cmd_syshdr = $(CONFIG_SHELL) $(syshdr) --abis $(abis) --emit-nr \