bpf: fix pseudo-c asm emitted for *mulsidi3_zeroextend

bpf: fix pseudo-c asm emitted for *mulsidi3_zeroextend

This patch fixes the pseudo-c BPF assembly syntax used for
*mulsidi3_zeroextend, which was being emitted as:

  rN *= wM

instead of the proper way to denote a mul32 in pseudo-C syntax:

  wN *= wM

Includes test.
Tested in bpf-unknown-none-gcc target in x86_64-linux-gnu host.

gcc/ChangeLog:

	* config/bpf/bpf.cc (bpf_print_register): Accept modifier code 'W'
	to force emitting register names using the wN form.
	* config/bpf/bpf.md (*mulsidi3_zeroextend): Force operands to
	always use wN written form in pseudo-C assembly syntax.

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/mulsidi3-zeroextend-pseudoc.c: New test.