tools/nolibc: prevent gcc from making memset() loop over itself

tools/nolibc: prevent gcc from making memset() loop over itself

[ Upstream commit 1bfbe1f3e96720daf185f03d101f072d69753f88 ]

When building on ARM in thumb mode with gcc-11.3 at -O2 or -O3,
nolibc-test segfaults during the select() tests. It turns out that at
this level, gcc recognizes an opportunity for using memset() to zero
the fd_set, but it miscompiles it because it also recognizes a memset
pattern as well, and decides to call memset() from the memset() code:

  000122bc <memset>:
     122bc:       b510            push    {r4, lr}
     122be:       0004            movs    r4, r0
     122c0:       2a00            cmp     r2, #0
     122c2:       d003            beq.n   122cc <memset+0x10>
     122c4:       23ff            movs    r3, #255        ; 0xff
     122c6:       4019            ands    r1, r3
     122c8:       f7ff fff8       bl      122bc <memset>
     122cc:       0020            movs    r0, r4
     122ce:       bd10            pop     {r4, pc}

Simply placing an empty asm() statement inside the loop suffices to
avoid this.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>