[RFA] Ensure GDB printf command can print convenience var strings without a target.

[Eli Zaretskii]

> From: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Cc: Philippe Waroquiers <philippe.waroquiers@skynet.be>
> Date: Mon, 10 Jun 2019 23:16:22 +0200
> 
> Without this patch, GDB printf command calls malloc on the target,
> writes the convenience var content to the target,
> re-reads the content from the target, and then locally printf the string.
> 
> This implies inferior calls, and does not work when there is no inferior,
> or when the inferior is a core dump.
> 
> With this patch, printf command can printf string convenience variables
> without inferior function calls.
> Ada string convenience variables can also be printed.
> 
> gdb/ChangeLog
> 
> 2019-06-10  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
> 
> 	* NEWS: Mention that GDB printf and eval commands can now print
> 	C-style and Ada-style convenience var strings without
> 	calling the inferior.
> 	* printcmd.c (printf_c_string): Locally print GDB internal var
> 	instead of transiting via the inferior.
> 	(printf_wide_c_string): Likewise.
> 
> 2019-06-10  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
> 
> 	* gdb.base/printcmds.exp: Test printing C strings and
> 	C wide strings convenience var without transiting via the inferior.
> ---
>  gdb/NEWS                             |   7 ++
>  gdb/printcmd.c                       | 143 +++++++++++++++++----------
>  gdb/testsuite/gdb.base/printcmds.exp |  39 ++++++++
>  3 files changed, 136 insertions(+), 53 deletions(-)

The NEWS part is OK, thanks.