bpf: allow exceeding max num of args in BPF when always_inline

BPF currently limits the number of registers used to pass arguments to
functions to five registers.  There is a check for this at function
expansion time.  However, if a function is guaranteed to be always
inlined (and its body never generated) by virtue of the always_inline
attribute, it can "receive" any number of arguments.

Tested in host x86_64-linux-gnu and target bpf-unknown-none.

gcc/ChangeLog

	* config/bpf/bpf.cc (bpf_function_arg_advance): Do not complain
	about too many arguments if function is always inlined.

gcc/testsuite/ChangeLog

	* gcc.target/bpf/diag-funargs-inline-1.c: New test.
	* gcc.target/bpf/diag-funargs.c: Adapt test.