[druntime]: Fix sys.posix.sys.types.nlink_t definition

[the-horo]

I've taken the definitions from glibc's
sysdeps/unix/sysv/linux/**/bits/typesizes.h.

For convenience:

alpha/bits/typesizes.h       #define  __NLINK_T_TYPE  __U32_TYPE
arm/bits/typesizes.h         #define  __NLINK_T_TYPE  __UWORD_TYPE
hppa/bits/typesizes.h        #define  __NLINK_T_TYPE  __UWORD_TYPE
m68k/bits/typesizes.h        #define  __NLINK_T_TYPE  __UWORD_TYPE
microblaze/bits/typesizes.h  #define  __NLINK_T_TYPE  __UWORD_TYPE
mips/bits/typesizes.h        #define  __NLINK_T_TYPE  __UWORD_TYPE
powerpc/bits/typesizes.h     #define  __NLINK_T_TYPE  __UWORD_TYPE
s390/bits/typesizes.h        #define  __NLINK_T_TYPE  __UWORD_TYPE
sh/bits/typesizes.h          #define  __NLINK_T_TYPE  __UWORD_TYPE
sparc/bits/typesizes.h       #define  __NLINK_T_TYPE  __U32_TYPE

bits/typesizes.h             #define  __NLINK_T_TYPE  __U32_TYPE

And x86:

 // sysdeps/unix/sysv/linux/x86/bits/typesizes.h
 #ifdef __x86_64__
 # define __NLINK_T_TYPE         __SYSCALL_ULONG_TYPE
 #else
 # define __NLINK_T_TYPE         __UWORD_TYPE
 #endif

[dlang-bot]

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Testing this PR locally

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dub run digger -- build "master + dmd#21770"

[the-horo]

Tested on aarch64 in ldc-developers/ldc#4971