bpf: add size threshold for inlining mem builtins

bpf: add size threshold for inlining mem builtins

BPF cannot fall back on library calls to implement memmove, memcpy and
memset, so we attempt to expand these inline always if possible.
However, this inline expansion was being attempted even for excessively
large operations, which could result in gcc consuming huge amounts of
memory and hanging.

Add a size threshold in the BPF backend below which to always expand
these operations inline, and introduce an option
-minline-memops-threshold= to control the threshold. Defaults to
1024 bytes.

gcc/

	* config/bpf/bpf.cc (bpf_expand_cpymem, bpf_expand_setmem): Do
	not attempt inline expansion if size is above threshold.
	* config/bpf/bpf.opt (-minline-memops-threshold): New option.
	* doc/invoke.texi (eBPF Options) <-minline-memops-threshold>:
	Document.

gcc/testsuite/

	* gcc.target/bpf/inline-memops-threshold-1.c: New test.
	* gcc.target/bpf/inline-memops-threshold-2.c: New test.