[libc++abi][NFCI] Consistently group new_handler, unexpected_handler and terminate_handler

Previously, the definitions of __cxa_terminate_handler and __cxa_unexpected_handler
(and their set_xxx_handler functions) were grouped together, but the
definition of __cxa_new_handler wasn't. This commit simply moves those
to the same file to treat all handlers consistently.

GitOrigin-RevId: 6714e1ce3b5ec02af2f9ab878bd10d273f02aa7e
2 files changed