bpf: use relative source file path in debug info

Enabling BTF leads clang to generate debug line info that includes the
absolute path to the source file. For C++ modules the
-fdebug-prefix-map flag is used to convert these to relative paths. Do
the same for BPF programs when BTF is enabled.

Test: build timeInState.c; strings shows only relative path to source
file
Bug: 238165437
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: Ie4ebc8f9a7d115baeddde5c7ac3ae710335ddf13