linux: Use statx for MIPSn64

MIPSn64 kernel ABI for legacy stat uses unsigned 32 bit for second
timestamp, which limits the maximum value to y2106.  This patch
make mips64 use statx as for 32-bit architectures.

Thie __cp_stat64_t64_statx is open coded, its usage is solely on
fstatat64 and it avoid the need to redefine the name for mips64
(which will call __cp_stat64_statx since its does not use
__stat64_t64 internally).