testsuite: add -O0 to Intel compilers if no 'optimize' option is given

icpx/icx give the following warning if '-g' is used without '-O'.

   icpx: remark: Note that use of '-g' without any optimization-level
   option will turn off most compiler optimizations similar to use of
   '-O0'; use '-Rno-debug-disables-optimization' to disable this
   remark [-Rdebug-disables-optimization]

The warning makes dejagnu think that compilation has failed.  E.g.:

  $ make check TESTS="gdb.cp/local.exp" RUNTESTFLAGS="CXX_FOR_TARGET='icpx' CC_FOR_TARGET=icx"
  ...
  gdb compile failed, icpx: remark: Note that use of '-g' without any optimization-level option will turn off most compiler optimizations similar to use of '-O0'; use '-Rno-debug-disables-optimization' to disable this remark [-Rdebug-disables-optimization]

                  === gdb Summary ===

  # of untested testcases         1

Furthermore, if no -O flag is passed, icx/icc optimize
the code by default.  This breaks assumptions in many GDB tests
that the code is unoptimized by default.  E.g.:

  $ make check TESTS="gdb.cp/cmpd-minsyms.exp" RUNTESTFLAGS="CXX_FOR_TARGET='icpx' CC_FOR_TARGET=icx"
  ...
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at 'GDB<int>::a() const'
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at 'GDB<int>::b() volatile'
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at 'GDB<int>::c() const volatile'
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at GDB<int>::operator ==
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at GDB<int>::operator==(GDB<int> const&)
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at GDB<char>::harder(char)
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at GDB<int>::harder(int)
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at "int GDB<char>::even_harder<int>(char)"
  FAIL: gdb.cp/cmpd-minsyms.exp: gdb_breakpoint: set breakpoint at GDB<int>::simple()

                  === gdb Summary ===

  # of expected passes            1
  # of unexpected failures        9

To fix both problems, pass the -O0 flag explicitly, if no optimization
option is given.

With this patch we get, e.g.:

  $ make check TESTS="gdb.cp/cmpd-minsyms.exp gdb.cp/local.exp" RUNTESTFLAGS="CXX_FOR_TARGET='icpx' CC_FOR_TARGET=icx"
  ...
                  === gdb Summary ===

  # of expected passes            19
  # of known failures             1

Approved-By: Tom Tromey <tom@tromey.com>