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

[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 d25a8dbc7c3 ("[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}.