[sanitizer] Fix vfork interception on loongarch64

Fix a brown paper bag error made by me in D129418.  I didn't set
ASAN_INTERCEPT_VFORK correctly for loongarch64, but created an all-zero
object for __interception::real_vfork.  This caused anything calling
vfork() to die instantly.

Fix this issue by setting ASAN_INTERCEPT_VFORK and remove the bad
all-zero definition.  Other ports have an all-zero common definition but
we don't need it at least for now.

And, enable ASAN vfork test for loongarch64 to prevent regression in the
future.

Differential Revision: https://reviews.llvm.org/D137160