always define empty PERFTOOLS_NOTHROW

Because somehow clang still builds "this function will not throw" code
even with noexcept. Which breaks performance of
tc_malloc/tc_new_nothrow. The difference with throw() seems to be just
which function is called when unexpected exception happens.

So we work around this sillyness by simply dropping any exception
specification when compiling tcmalloc.
1 file changed