[gdb/testsuite] Fix incomplete regexps in step-precsave.exp

[gdb/testsuite] Fix incomplete regexps in step-precsave.exp

The commit 68f7d34dd50 "[gdb/testsuite] Add KFAIL for missing support of
reverse-debugging of vmovd" rewrites a gdb_test into a gdb_test_multiple but
forgets to add the $gdb_prompt part in the regexp.

Add the missing parts of the regexps.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-09-27  Tom de Vries  <tdevries@suse.de>

	* gdb.reverse/step-precsave.exp: Add missing $gdb_prompt in regexps.