linux: Consolidate fxstatat{64}

linux: Consolidate fxstatat{64}

The LFS support is implemented on fxstat64.c, instead of fxstat.c for
64-bit architectures.  The fxstatat.c implements the non-LFS and it is
a no-op for !XSTAT_IS_XSTAT64.

The generic non-LFS implementation handles two cases:

  1. New kABIs which uses generic pre 64-bit time Linux ABI (csky and
     nios): it issues __NR_fstatat64 plus handle the overflow on st_ino,
     st_size, or st_blocks.  It only handles _STAT_VER_KERNEL.

  2. Old kABIs with old non-LFS support (arm, i386, hppa, m68k, mips32,
     microblaze, s390, sh, powerpc, and sparc32).  it issues
     __NR_fstatat64 and convert to non-LFS stat struct based on the
     version.

Also non-LFS mips64 is an outlier and it has its own implementation
since _STAT_VER_LINUX requires a different conversion function (it
uses the kernel_stat as the sysissues argument since its exported ABI
is different than the kernel one for both non-LFS and LFS
implementation).

The generic LFS implementation handles multiple cases:

  1. XSTAT_IS_XSTAT64 being 1:

    1.1. 64-bit kABI (aarch64, ia64, powerpc64*, s390x, riscv64, and
         x86_64): it issues __NR_newfstatat for _STAT_VER_KERNEL or
         _STAT_VER_LINUX.

    1.2. 64-bit kABI outlier (sparc64): it issuess fstatat64 with a
         temporary stat64 and convert to output stat64 based on the
         input version (and using a sparc64 specific __xstat32_conv).

    1.3. New 32-bit kABIs with only 64-bit time_t support (arc and
	 riscv32): it issues __NR_statx and covert to struct stat64.

  2. Old ABIs with XSTAT_IS_XSTAT64 being 0 (arm, csky, i386, hppa, m68k,
     microblaze, mips32, nios2, sh, powerpc32, and sparc32): it issues
     __NR_fstat64.

Also, two special cases requires specific implementations:

  1. alpha: it uses the __NR_fstatat64 syscall instead.

  2. mips64: as for non-LFS implementation its ABIs differ from
     glibc exported one, which requires an specific conversion
     function to handle the kernel_stat.

Checked with a build for all affected ABIs. I also checked on x86_64,
i686, powerpc, powerpc64le, sparcv9, sparc64, s390, and s390x.

Reviewed-by: Lukasz Majewski <lukma@denx.de>