libcsupport: POSIX conformance for _exit()

According to POSIX the _exit() function shall not call functions
registered with atexit() nor any registered signal handlers.

See also tests libtests/exit01 and libtests/exit02.

Make libc_wrapup() static.  Remove out of date comments.  Remove
superfluous declarations, defines and includes.
bsp/mpc55xx: Add MMU tag translate initializer