lib/string.c: Use freestanding environment

gcc can transform the loop in a naive implementation of memset/memcpy
etc into a call to the function itself.  This optimization is enabled by
-ftree-loop-distribute-patterns.

This has been the case for a while, but gcc-10.x enables this option at
-O2 rather than -O3 as in previous versions.

Add -ffreestanding, which implicitly disables this optimization with
gcc.  It is unclear whether clang performs such optimizations, but
hopefully it will also not do so in a freestanding environment.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>