PR23860: additional ugly stack/clobber protection for strings

PR23860: additional ugly stack/clobber protection for strings

In addition to prior commit, emit_string_copy() does not work for
overlapping source/destination. So, make sure strings are not
allocated in a way which overlaps map key/value arguments.

This increases space pressure, inducing a couple of bpf-asm.exp
testcase failures.

* bpf-internal.h (value::format_str): New flag.
(value::value): Take format_str flag.
(value::mk_str): Take format_str flag.
(program::format_map): New field, caches format_str separately.
(program::new_str): Take format_str flag.
* bpf-base.cxx (program::new_str): Cache format_str separately.
* bpf-opt.cxx (alloc_literal_str): Store non-format str in lower half.
* bpf-translate.cxx (emit_string_copy): Comment -- doesn't support overlap.
(emit_string_copy): DEBUG_CODEGEN -- identify if zero-padding was done.
(emit_print_format): Set format_str flag.