r600g: Correctly initialize the shader key, v2

Assigning a struct only copies the members - any padding is left as is.

Thus this code:

struct foo_t foo;
foo = bar;

leaves the padding of foo intact, ie uninitialized random garbage.

This patch fixes constant shader recompiles by initializing the struct
to zero. For completeness, memcpy is used to copy the key to the shader
struct.

NOTE: This is a candidate for the stable branches.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
(cherry picked from commit e495d88453076a4fe552502d66d6a3869ab70e4a)
targets/dri-i915: Force c++ linker in all cases

NOTE: This is a candidate for the 9.1 branch.
Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=461696
Reviewed-by: Andreas Boll <andreas.boll.dev@gmail.com>
(cherry picked from commit 121c2c898368ec8de54d230b7db0316695e63a83)