[hwasan] Move __hwasan_thread_enter/__hwasan_thread_exit out of namespace (#72123)

Due to a GCC bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25940),
GCC doesn't consider extern "C" functions with the same name but
different namespace to be the same. As such, the default visibility
attribute (on a declaration outside the namespace) doesn't get applied
to the definition in the namespace and the symbol is not exported.

This came up as an ABI diff when switching between gcc and clang for
compiling compiler-rt.