s390x regtest: Adjust op00 and op_exception tests for Clang

s390x regtest: Adjust op00 and op_exception tests for Clang

As part of fixing Bug 465782, adjust the op00 and op_exception tests.

The op00 test case doesn't compile with clang because the .hword assembler
directive is not known to llvm-as.  When replacing it with .short, the
test case fails because clang/llvm-as generates different line number
information from gcc/gas.  Using gcc, Valgrind's error message locates the
unrecognised instruction

   at 0x........: main (op00.c:3)

whereas with clang, it shows:

   at 0x........: main (op00.c:4)

The asm statement in op00.c is indeed located at line 4, so when compiling
with gcc, the indicated line number is just wrong.  This is because gas
only updates line number information for instruction directives, not for
directives like `.short' as used here.  See also the discussion in
https://sourceware.org/bugzilla/show_bug.cgi?id=30206

The same applies to the op_exception test case, where `.long' is used to
emit illegal instructions.

Make these test cases independent from this difference, by replacing the
`.short' and `.long' directives by equivalent `.insn' directives.  Also
drop the unnecessary duplication of op00.stderr.exp.