gdb/testsuite: make test names unique in gdb.arch/*.exp

gdb/testsuite: make test names unique in gdb.arch/*.exp

Make the test names unique in gdb.arch/*.exp by either modifying the
test names or using with_test_prefix.

I have also fixed a typo 'forth' -> 'fourth' throughout gdb.arch/*.

Finally, I replaced code like this:

  gdb_test "break [gdb_get_line_number "first breakpoint here"]" \
           "Breakpoint .* at .*${srcfile}.*" \
           "set first breakpoint in main"

With this:

  gdb_breakpoint [gdb_get_line_number "first breakpoint here"]

In those files that I was already modifying for the other reasons
given above.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-byte.exp: Make test names unique, use
	gdb_breakpoint, and fix typo 'forth' -> 'fourth'.
	* gdb.arch/amd64-dword.exp: Likewise.
	* gdb.arch/amd64-pseudo.c: Fix typo 'forth' -> 'fourth'.
	* gdb.arch/amd64-stap-special-operands.exp: Make test names
	unique.
	* gdb.arch/amd64-tailcall-ret.exp: Likewise.
	* gdb.arch/amd64-word.exp: Make test names unique, use
	gdb_breakpoint, and fix typo 'forth' -> 'fourth'.
	* gdb.arch/i386-byte.exp: Make test names unique, use
	gdb_breakpoint.
	* gdb.arch/i386-word.exp: Likewise.