gdbserver: i387_cache_to_xsave: fix copy dest of zmm registers

gdbserver: i387_cache_to_xsave: fix copy dest of zmm registers

On a machine with AVX512 support (AMD EPYC 9634), I see these failures:

    $ make check TESTS="gdb.arch/i386-avx512.exp" RUNTESTFLAGS="--target_board=native-gdbserver"
    ...
    FAIL: gdb.arch/i386-avx512.exp: check contents of zmm_data[16] after writing ZMM regs
    FAIL: gdb.arch/i386-avx512.exp: check contents of zmm_data[17] after writing ZMM regs
    FAIL: gdb.arch/i386-avx512.exp: check contents of zmm_data[18] after writing ZMM regs
    ...

The problem can be reduced to:

    (gdb) print $zmm16.v8_int64
    $1 = {0, 0, 0, 0, 0, 0, 0, 0}
    (gdb) print $zmm16.v8_int64 = {11,22,33,44,55,66,77,88}
    $2 = {11, 22, 33, 44, 55, 66, 77, 88}
    (gdb) print $zmm16.v8_int64
    $3 = {11, 22, 33, 44, 55, 66, 77, 88}
    (gdb) step
    5               ++x;
    (gdb) print $zmm16.v8_int64
    $4 = {11, 22, 77, 88, 0, 0, 0, 0}

Writing to the local regcache in GDB works fine, but the writeback to
gdbserver (which happens when resuming / stepping) doesn't work (the
code being stepped doesn't touch AVX registers, so we don't expect the
value of zmm16 to change when stepping).

The problem is on the gdbserver side, the zmmh and ymmh portions of the
zmm register are not memcpied at the right place in the xsave buffer.  Fix
that.  Note now how the two modified memcpy calls match the memcmp calls
just above them.

With this patch, gdb.arch/i386-avx512.exp passes completely for me.

Change-Id: I22c417e0f5e88d4bc635a0f08f8817a031c76433
Reviewed-by: John Baldwin <jhb@FreeBSD.org>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30818