Revert "mjit_worker.c: try passing -nostdlib to AIX"

	This reverts commit b32fb230.

	This didn't work on AIX.

	mjit_worker.c: do not use GCC_NOSTDLIB_FLAGS for OpenBSD

	OpenBSD's GCC compiler has local extensions that break when
	`-nostartfiles -nodefaultlibs -nostdlib` is used.

	From: Jeremy Evans <merch-redmine@jeremyevans.net>

	[Bug #15548]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e