bpf: add inline memmove and memcpy expansion

bpf: add inline memmove and memcpy expansion

BPF programs are not typically linked, which means we cannot fall back
on library calls to implement __builtin_{memmove,memcpy} and should
always expand them inline if possible.

GCC already successfully expands these builtins inline in many cases,
but failed to do so for a few for simple cases involving overlapping
memmove in the kernel BPF selftests and was instead emitting a libcall.

This patch implements a simple inline expansion of memcpy and memmove in
the BPF backend in a verifier-friendly way, with the caveat that the
size must be an integer constant, which is also required by clang.

gcc/

	* config/bpf/bpf-protos.h (bpf_expand_cpymem): New.
	* config/bpf/bpf.cc: (emit_move_loop, bpf_expand_cpymem): New.
	* config/bpf/bpf.md: (cpymemdi, movmemdi): New define_expands.

gcc/testsuite/

	* gcc.target/bpf/memcpy-1.c: New test.
	* gcc.target/bpf/memmove-1.c: New test.
	* gcc.target/bpf/memmove-2.c: New test.