gdb, btrace: Fix rn-dl-bind.exp for new icx remark.

gdb, btrace: Fix rn-dl-bind.exp for new icx remark.

When running the test with the latest Intel compiler:

Running gdb/gdb/testsuite/gdb.btrace/rn-dl-bind.exp ...
gdb compile failed, icpx: warning: treating 'c' input as 'c++' when
in C++ mode, this behavior is deprecated [-Wdeprecated]

The test doesn't seem to test something specifically for C++,
so I removed the C++ compilation option. Alternatively we could rename
rn-dl-bind.exp.c to rn-dl-bind.exp.cc.

Signed-off-by: Felix Willgerodt <felix.willgerodt@intel.com>