scylla-gdb: add printing registers from seastar::thread
scylla-gdb: add printing registers from seastar::thread

`scylla thread` command is extended with a non-intrusive
option for dumping saved registers from the jmp_buf structure
in an unmangled form.
It can later be useful, e.g. for peeking into thread's instruction
pointer or reasoning about its stack.

Example debugging session:
(gdb) scylla threads
[shard 1] (seastar::thread_context*) 0x6010000d9e00, stack: 0x601004f00000
[shard 1] (seastar::thread_context*) 0x6010000daf00, stack: 0x601004e00000

(gdb) scylla thread --print-regs 0x6010000d9e00
rbx: 0x601004f1fd00
rbp: 0x601004f1fc20
r12: 0x6010000d9e20
r13: 0x6010002a3190
r14: 0x601004f1fd08
r15: 0x6010000d9e10
rsp: 0x601004f1fbb0
rip: 0x2f0aea6

(gdb) disassemble 0x2f0aea6
Dump of assembler code for function _ZN7seastar12jmp_buf_link10switch_outEv:
   0x0000000002f0ae90 <+0>:	push   %rax
   0x0000000002f0ae91 <+1>:	mov    0xc8(%rdi),%rax
   0x0000000002f0ae98 <+8>:	mov    %rax,%fs:0xfffffffffffe5dc8
   0x0000000002f0aea1 <+17>:	call   0x30333d0 <_setjmp@plt>
   0x0000000002f0aea6 <+22>:	test   %eax,%eax
   0x0000000002f0aea8 <+24>:	je     0x2f0aeac <_ZN7seastar12jmp_buf_link10switch_outEv+28>
   0x0000000002f0aeaa <+26>:	pop    %rax
   0x0000000002f0aeab <+27>:	ret
   0x0000000002f0aeac <+28>:	mov    %fs:0xfffffffffffe5dc8,%rdi
   0x0000000002f0aeb5 <+37>:	mov    $0x1,%esi
   0x0000000002f0aeba <+42>:	call   0x30333c0 <longjmp@plt>
End of assembler dump.
Message-Id: <0a3b5b9bbbf03a84c0e1208f97f5ff3efb2552ec@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla