[gdb/testsuite] Use require in gdb.dwarf2/assign-variable-value-to-register.exp

[gdb/testsuite] Use require in gdb.dwarf2/assign-variable-value-to-register.exp

In test-case gdb.dwarf2/assign-variable-value-to-register.exp a return is
missing here after the unsupported:
...
if { ![is_x86_64_m64_target] } {
    unsupported "unsupported architecture"
}
...
and consequently on aarch64-linux I ran into an UNSUPPORTED followed by 3
FAILs.

Fix this by simply using require:
...
require is_x86_64_m64_target
...

Tested on x86_64-linux and aarch64-linux.