powerpc/vdso: Refactor CFLAGS for CVDSO build

[ Upstream commit a6b67eb0 ]

In order to avoid two much duplication when we add new VDSO
functionnalities in C like getrandom, refactor common CFLAGS.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Stable-dep-of: d677ce52 ("powerpc/vdso: Drop -mstack-protector-guard flags in 32-bit files with clang")
Signed-off-by: Sasha Levin <sashal@kernel.org>