BACKPORT: arm64: Kconfig: Make CONFIG_COMPAT_VDSO a proper Kconfig option

CONFIG_COMPAT_VDSO is defined by passing '-DCONFIG_COMPAT_VDSO' to the
compiler when the generic compat vDSO code is in use. It's much cleaner
and simpler to expose this as a proper Kconfig option (like x86 does),
so do that and remove the bodge.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 7c4791c9efca8c105a86022f7d5532aeaa819125)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I33c90b5bee896a41cfd38738a5d19f590a6010c4