gdb/testsuite: updates for gdb.arch/{amd64,i386}-disp-step-self-call.exp

This commit:

  commit cf141dd8
  Date:   Wed Feb 22 12:15:34 2023 +0000

      gdb: fix reg corruption from displaced stepping on amd64

Added two test scripts gdb.arch/amd64-disp-step-self-call.exp and
gdb.arch/i386-disp-step-self-call.exp.  These scripts contained a test
that included a stack address in the test name, this makes it harder
to compare results between runs.

This commit gives the tests proper names that doesn't include an
address.

Also in gdb.arch/i386-disp-step-self-call.exp I noticed that we were
writing 8-bytes rather than 4 in order to clear the return address
entry on the stack.  This is also fixed in this commit.