gdb/testsuite: fix gdb.trace/signal.exp on x86

Patch

  202be274a41a ("opcodes/i386: remove trailing whitespace from insns with zero operands")

causes this regression:

  FAIL: gdb.trace/signal.exp: find syscall insn in kill

It's because the test still expects to match a whitespace after the
instruction, which the patch mentioned above removed.  Remove the
whitespaces for the regexp.

Change-Id: Ie194273cc942bfd91332d4035f6eec55b7d3a428