Disallow '_' in bpf source name

Current design:
1. The bpf compiled object name is derived from the source name
   (e.g. foo.c -> foo.o).
2. Full bpf program/map name are concatenated by object name + '_' +
   program/map name in run-time. (e.g. obj name: x.o; program name: y_z;
   full bpf program name will be x_y_z)

Issue:
x.o with map y_z and x_y.o with map z can cause naming collision in
run-time, since both result in x_y_z. This commit prevents it from
happening with a build-time check.

Bug: 236706995
Test: m
Change-Id: Ic03bfcf07a5748ed63246b71d5ae8de0405e658a