gdb: fix undefined behavior reported in copy_bitwise

gdb: fix undefined behavior reported in copy_bitwise

gdb version 9.1, built with clang 8.0.0 on Ubuntu 18.04 (x86_64);
--enable-ubsan (for clang's undefined behavior sanitizer)

Executing command; `maint selftest copy_bitwise` bombs in runtime error:
../../gdb/utils.c:3432:28: runtime error: left shift of negative value -1

Closer look reveals the offending shift: `(~0 << nbits)`, apparently 0
is treated as signed int, resulting in negative complement. Explicitly
stating it unsigned 0U  fixes it and the `copy_bitwise` test passes
ok.