gas: use memmove instead of bogus memcpy.

partial_where points into the buffer that begins with buffer_start
so we need to use memmove() to handle the overlap.
Sourceware-PR 11456.

Obtained from:	OpenBSD (CVS rev. 1.2)
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276228 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f