BPF: generate proper BTF for globals with WeakODRLinkage

For a global weak symbol defined as below:
  char g __attribute__((weak)) = 2;
LLVM generates an allocated global with WeakAnyLinkage,
for which BPF backend generates proper BTF info.

For the above example, if a modifier "const" is added like
  const char g __attribute__((weak)) = 2;
LLVM generates an allocated global with WeakODRLinkage,
for which BPF backend didn't generate any BTF as it
didn't handle WeakODRLinkage.

This patch addes support for WeakODRLinkage and proper
BTF info can be generated for weak symbol defined with
"const" modifier.

Differential Revision: https://reviews.llvm.org/D100362