Avoid undefined behaviour in gas's rotate_left() macro for n == 0.
Otherwise, clang can effectively remove the first iteration of the for
loops where this macro is invoked, and as a result, "cmp r0, #99" fails
to assemble.

Obtained from:	joerg at netbsd
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274856 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f