ANDROID: arm64: vdso32: support user-supplied flags

This introduces a new environment variable, KCPPFLAGS_COMPAT.

One use-case is to ensure -ffile-prefix-map is passed to the arm32
compiler to normalise compilation directory and make the ELF build ID
reproducible.

Bug: 345452375
Change-Id: I6ae1df58172f4dadeac1dbbee2e3241b704a9256
Signed-off-by: Giuliano Procida <gprocida@google.com>