[gdb/tdep] Handle si_addr_bnd in compat_siginfo_from_siginfo

[gdb/tdep] Handle si_addr_bnd in compat_siginfo_from_siginfo

When running test-case gdb.arch/i386-mpx-sigsegv.exp with target board
unix/-m32, we run into:
...
(gdb) continue^M
Continuing.^M
Saw a #BR! status 1 at 0x8048c2d^M
^M
Program received signal SIGSEGV, Segmentation fault^M
Upper bound violation while accessing address 0x0804c15c^M
Bounds: [lower = 0x00000000, upper = 0x00000000].^M
0x08048a4f in lower (p=0x804c160, a=0x804c180, b=0x804c1a0, c=0x804c1c0, \
  d=0x804c1e0, len=1) at i386-mpx-sigsegv.c:79^M
79        value = *(p - len);^M
(gdb) FAIL: gdb.arch/i386-mpx-sigsegv.exp: MPX signal segv Lower: 0
...

The problem is that lower and upper in the Bounds message are 0x0, which is
caused by $_siginfo._sifields._sigfault._addr_bnd.{_lower,_upper} evaluating
to 0x0.

Fix this by copying the si_lower/si_upper fields in
compat_siginfo_from_siginfo.

Tested on x86_64-linux, with target board unix/-m32.

gdb/ChangeLog:

2021-01-18  Tom de Vries  <tdevries@suse.de>

	PR tdep/27172
	* nat/amd64-linux-siginfo.c (cpt_si_lower, cpt_si_upper, SEGV_BNDERR):
	New macro.
	(compat_siginfo_from_siginfo): Copy cpt_si_lower and cpt_si_upper
	for SEGV_BNDERR.