[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.arch/powerpc*.exp

Fix clean_restart <absolute filename> in gdb.arch/powerpc*.exp.

Likewise in gdb.arch/ppc*.exp, gdb.arch/altivec*.exp, gdb.arch/e500*.exp and
gdb.arch/vsx*.exp.

Tested on ppc64le-linux.

The following test-cases only run on 32-bit ppc:
- gdb.arch/e500-abi.exp
- gdb.arch/e500-prologue.exp
- gdb.arch/e500-regs.exp
- gdb.arch/powerpc-aix-prologue.exp
- gdb.arch/powerpc-prologue.exp
- gdb.arch/powerpc-prologue-frame.exp
- gdb.arch/powerpc-trap.exp
so these haven't been tested.