Thank you for the feedbacks.  I updated the patch adding negative repeat count
support to the 'x' command instead of a new format letter.  Also, this patch
includes the update of document and NEWS file.

With this, we can specify a negative repeat count for all the formats in the 'x'
command to examine memory backward.

Here's the example output:
(gdb) bt
#0  Func1 (n=42, p=0x40432e "hogehoge") at main.cpp:5
#1  0x00000000004041fa in main (argc=1, argv=0x7fffffffdff8) at main.cpp:19
(gdb) x/-4i 0x4041fa
   0x4041e5 <main(int, char**)+11>:	mov    %rsi,-0x10(%rbp)
   0x4041e9 <main(int, char**)+15>:	lea    0x13e(%rip),%rsi        # 0x40432e
   0x4041f0 <main(int, char**)+22>:	mov    $0x2a,%edi
   0x4041f5 <main(int, char**)+27>:	callq  0x404147 <Func1(int, char const*)>
(gdb) x/-4xw 0x404200
0x4041f0 <main(int, char**)+22>:	0x00002abf	0xff4de800	0x76e8ffff	0xb8ffffff
(gdb) x/-4
0x4041e0 <main(int, char**)+6>:	0x7d8910ec	0x758948fc	0x358d48f0	0x0000013e

Tested on i386-linux and amd64-linux. I wrote a new testcase 'examine-backward'
under gdb.base.  It mainly focuses on x/-#s and x/-#i.  For disassembler
testing, the new testcase covers only i386 and amd64, which are skipped onother
platforms.

gdb/ChangeLog:

	* NEWS: Mention that GDB now supports a negative repeat count in
	the 'x' command.
	* printcmd.c (decode_format): Allow '-' in the parameter
	"string_ptr" to accept a negative repeat count.
	(null_fprintf): New function.
	(null_print_address): New function.
	(find_instruction_backward): New function.
	(read_memory_backward): New function.
	(integer_is_zero): New function.
	(find_string_backward): New function.
	(do_examine): Use new functions to examine memory backward.

gdb/doc/ChangeLog:

	* gdb.texinfo (Examining Memory): Document negative repeat
	count in the 'x' command.

gdb/testsuite/ChangeLog:

	* gdb.base/examine-backward.c: New file.
	* gdb.base/examine-backward.exp: New file.