Fix gdb.trace testcase build failures with Clang

Fix gdb.trace testcase build failures with Clang

25 gdb.trace tests failed to build on x86 with Clang because the
x86_trace_dummy function is optimized out, causing the builds to
fail with variations on the following error:

  gdb compile failed, /usr/bin/ld:
    /gdbtest/build/gdb/testsuite/outputs/gdb.trace/backtrace/backtrace0.o:
    in function `main':
    /gdbtest/src/gdb/testsuite/gdb.trace/actions.c:146: undefined
    reference to `x86_trace_dummy'
    clang-12: error: linker command failed with exit code 1

This commit adds __attribute__ ((used)) to x86_trace_dummy to prevent
this.

gdb/testsuite/ChangeLog:

	* gdb.trace/trace-common.h (x86_trace_dummy): Add
	__attribute__ ((used)).