Wrap linux specific cachestat syscall

Wrap linux specific cachestat syscall

cachestat takes an fd, cstat_range and flags arguments and
writes out page cache statistics via the cstat struct.

Declare a sys_cachestat wrapper in priv_syswrap-linux.h and hook it
for {amd64,arm,arm64,mips64,nanomips,ppc32,ppc64,riscv64,s390x,x86}-linux
using LINXY with PRE/POST handlers in syswrap-linux.c.

Define __NR_cachestat for amd64, arm, arm64, mips32, mips64, nanomips,
ppc32, ppc64, riscv64, s390x, and x86.

https://bugs.kde.org/show_bug.cgi?id=501741