bpf: add inline memset expansion

bpf: add inline memset expansion

Similar to memmove and memcpy, the BPF backend cannot fall back on a
library call to implement __builtin_memset, and should always expand
calls to it inline if possible.

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

gcc/
	* config/bpf/bpf-protos.h (bpf_expand_setmem): New prototype.
	* config/bpf/bpf.cc (bpf_expand_setmem): New.
	* config/bpf/bpf.md (setmemdi): New define_expand.

gcc/testsuite/
	* gcc.target/bpf/memset-1.c: New test.

(cherry picked from commit eae6b63b5b5426f943f58b5ae0bf0a6068ca8ad6)