[gdb/testsuite] Add gdb.arch/ppc64-break-on-_exit.exp

Add a regression test-case for commit a50bdb99 "[gdb/tdep, rs6000] Don't
skip system call in skip_prologue":
- set a breakpoint on a local copy of glibc's _exit, and
- verify that it triggers.

The test-case uses an assembly file by default, but also has the possibility
to use a C source file instead.

Tested on ppc64le-linux.  Verified that the test-case fails without
aforementioned commit, and passes with the commit.  Both with assembly
and C source.