gdb, testsuite: Enable testcases that suppress specific warnings, for icc/icx.

gdb, testsuite: Enable testcases that suppress specific warnings, for icc/icx.

To cite gdb.exp:
Some C/C++ testcases unconditionally pass -Wno-foo as additional
options to disable some warning.  That is OK with GCC, because
by design, GCC accepts any -Wno-foo option, even if it doesn't
support -Wfoo.  Clang however warns about unknown -Wno-foo by
default, unless you pass -Wno-unknown-warning-option as well.
We do that here, so that individual testcases don't have to
worry about it.

This patch adds the same option that already exists for clang for icx and
adds the equivalent icc option.