amd/int0x80 test: fix inline asm for gcc

[rlibby]

Summary

Gcc complained about the unused %0 asm argument.

Fixes: 2c2ec6bbc9cc ("tests/sys/arch/amd64: add a program to check INT $0x80 behavior on amd64")

---

Gcc's error is not super clear but removing the unused input operand solved it.

/usr/src/freebsd/tests/sys/arch/amd64/int0x80.c: In function 'fire':
/usr/src/freebsd/tests/sys/arch/amd64/int0x80.c:55:13: error: '%' constraint used with last operand
   55 |             : "rdx", "memory", "cc");
      |             ^

Test Plan

make buildworld && env MAKEOBJDIRPREFIX=/usr/obj/gcc14 CROSS_TOOLCHAIN=amd64-gcc14 make buildworld

Checked object dump for both clang and gcc builds for

mov $14,%eax
int $0x80

Ran test with default clang build:

kyua test -k /usr/tests/sys/arch/Kyuafile