Remove -Wno-thread-safety-negative

-Wthread-safety-negative was disabled because it requires locks to
support an operator!(), and std::mutex in libc++ does not.  Disabling
it everywhere was preventing it being used for modules that want to
opt in to using it and provide their own lock that supports
operator!().  Modules that use -Weverything should either stop using
 -Weverything (preferred), or add -Wno-thread-safety-negative.

Bug: 143713277
Test: m native
Change-Id: I0b84679a806a21b16cabe7f739fde61c3efe2cbe