Fixed for eliminating picolibc in package writing

[kaolpr]

While trying to make a reproducible LiteX build using Nix I found that libc Makefile copies CPU family files to $PICOLIBC_DIRECTORY/newlib/libc/machine/$(CPUFAMILY)/. This directory is a package directory and should not be altered during build (what was the cause of build failure due to insufficient permissions in my case).

This PR suggests a walk around that copies all contents of $PICOLIBC_DIRECTORY to picolibc_src in current build directory and copies libc CPU family files there.