[gdb/testsuite] Use gdb_load in gdb.rocm/break-kernel-no-debug-info.exp

[gdb/testsuite] Use gdb_load in gdb.rocm/break-kernel-no-debug-info.exp

The gdb.rocm/break-kernel-no-debug-info.exp builds a testcase without
debug info, and loads the binary in GDB with:

    gdb_test "file $::binfile" ".*No debugging symbols.*" "load file"

On some configurations (SLES with glibc-devel installed), the compiler
links /usr/lib64/crit1.o (which contains some debug information) into
the final binary.  As a result, the binary contains "some" debug
information, which makes the above pattern fail.  The testcase is still
valid as we only really need the GPU code object (embedded in the main
binary) to not contain debug information, so this patch proposes to
relax this first step by using "gdb_load" instead.

Tested on x86_64-linux + AMDGPU gfx1031.

Change-Id: Id903f62e170af69c001b39f4681602a54e2fdaf1
Approved-By: Tom Tromey <tom@tromey.com>