Add Valgrind's implementation of memmove to avoid link issue

One of the recent changes, r2682 (Make HReg a struct), caused a build
break on several x86_64 and MIPS build bots/platforms that used older
gcc versions. The issue was that compilers generated calls to memmove,
and since it was built with -nodefaultlibs, the entry could not be
resolved. The fix wraps VG_(memmove) in memmove().


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13288 a5019735-40e9-0310-863c-91ae7b9d1cf9