When MAKEOBJDIRPREFIX points to a case-insensitive file system, the
build can break when different source files create the same object
files (case-insensitivity speaking). This is the case for _Exit.c
and _exit.s. Compile _Exit.c as C99_Exit.c

Reviewed by: sjg@
MFC after: completion
Sponsored by: Bracket Computing
Differential Revision: https://reviews.freebsd.org/D7893