hardcoding NR_gettid may be bad idea.
```
[11:29] *@* (master) ~/contrib-src/linux-stable $ git grep _NR_gettid
arch/alpha/include/uapi/asm/unistd.h:#define __NR_gettid                        378
arch/arm/include/uapi/asm/unistd.h:#define __NR_gettid                  (__NR_SYSCALL_BASE+224)
arch/avr32/include/uapi/asm/unistd.h:#define __NR_gettid                176
arch/blackfin/include/uapi/asm/unistd.h:#define __NR_gettid             224
arch/cris/include/uapi/asm/unistd.h:#define __NR_gettid             224
arch/frv/include/uapi/asm/unistd.h:#define __NR_gettid          224
arch/ia64/include/uapi/asm/unistd.h:#define __NR_gettid                 1105
arch/m32r/include/uapi/asm/unistd.h:#define __NR_gettid         224
arch/m68k/include/uapi/asm/unistd.h:#define __NR_gettid         221
arch/microblaze/include/uapi/asm/unistd.h:#define __NR_gettid           224 /* ok */
arch/mips/include/uapi/asm/unistd.h:#define __NR_gettid                 (__NR_Linux + 222)
arch/mips/include/uapi/asm/unistd.h:#define __NR_gettid                 (__NR_Linux + 178)
arch/mips/include/uapi/asm/unistd.h:#define __NR_gettid                 (__NR_Linux + 178)
arch/mn10300/include/uapi/asm/unistd.h:#define __NR_gettid              224
arch/parisc/include/uapi/asm/unistd.h:#define __NR_gettid             (__NR_Linux + 206)
arch/powerpc/include/uapi/asm/unistd.h:#define __NR_gettid              207
arch/s390/include/uapi/asm/unistd.h:#define __NR_gettid         236
arch/sh/include/uapi/asm/unistd_32.h:#define __NR_gettid                224
arch/sh/include/uapi/asm/unistd_64.h:#define __NR_gettid                252
arch/sparc/include/uapi/asm/unistd.h:#define __NR_gettid             143 /* ENOSYS under SunOS                          */
arch/xtensa/include/uapi/asm/unistd.h:#define __NR_gettid                               127
include/uapi/asm-generic/unistd.h:#define __NR_gettid 178
include/uapi/asm-generic/unistd.h:__SYSCALL(__NR_gettid, sys_gettid)
```