re PR go/65462 (Use of 'go get' with gccgo is not finding dependencies correctly)

	PR go/65462
cmd: Fix dependencies for 'go get' with gccgo

Problem described in GCC BZ 65462.
Generate the list of the standard GO package names based on what was built into libgo in the libgo Makefile.
Change the var name from reqPkgSrc to reqStdPkgSrc to clarify it only affects standard GO packages.
Skip the attempted loading of a package only if it is a standard GO package and the flag is set indicating its source is not required to be available.
This requires a corresponding change to gotools to build and link in the new file containing the list of standard GO package names that was generated by the libgo Makefile.

gotools/:
	PR go/65462
	* Makefile.am (go_cmd_go_files): Add $(libgodir)/zstdpkglist.go.
	* Makefile.in: Rebuild.

From-SVN: r221643