gdbsupport: constify some return values in print-utils.{h,cc}

gdbsupport: constify some return values in print-utils.{h,cc}

There is no reason the callers of these functions need to change the
returned string, so change the `char *` return types to `const char *`.

Update a few callers to also use `const char *`.

Change-Id: I94adff574d5e1b326e8cc688cf1817a15b408b96
Approved-By: Tom Tromey <tom@tromey.com>