[gdb/testsuite] Fix gdb.base/unwind-on-each-insn.exp for -m32

With test-case gdb.base/unwind-on-each-insn.exp and target board unix/-m32, I
now get:
...
 # of expected passes            25
...
instead of:
...
 # of expected passes            133
...
as I used to get before commit d25a8dbc7c ("[gdb/testsuite] Allow debug
srcfile2 in gdb.base/unwind-on-each-insn.exp"), due to the test-case trying to match
"rip = " and info frame printing "eip = " instead.

Fix this by dropping "rip" from the regexp.

Tested on x86_64-linux, target boards unix/{-m64,-m32}.

gdb/testsuite/gdb.base/unwind-on-each-insn.exp

@@ -146,7 +146,7 @@ proc do_test { function step_cmd } {
 
 	    set in_fn 0
 	    gdb_test_multiple "info frame" "" {
-		-re -wrap "rip = $::hex in ${function}( \\(.*\\))?;.*" {
+		-re -wrap " = $::hex in ${function}( \\(.*\\))?;.*" {
 		    set in_fn 1
 		}
 		-re -wrap "" {}