tools/nolibc: Replace `asm` with `__asm__`

Replace `asm` with `__asm__` to support compilation with -std flag.
Using `asm` with -std flag makes GCC think `asm()` is a function call
instead of an inline assembly.

GCC doc says:

  For the C language, the `asm` keyword is a GNU extension. When
  writing C code that can be compiled with `-ansi` and the `-std`
  options that select C dialects without GNU extensions, use
  `__asm__` instead of `asm`.

Link: https://gcc.gnu.org/onlinedocs/gcc/Basic-Asm.html


Reported-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>