arch-syscall-check: verify #ifdef __NR_ guards match __SNR_ define names

[rawrmonster17]

Problem

src/arch-syscall-check already verifies that the set of syscall names in
syscalls.csv matches the set of __SNR_ defines in seccomp-syscalls.h.
However it does not verify that each #ifdef __NR_<name> guard immediately
preceding a #define __SNR_<name> uses the same syscall name as the
define it guards.

A mismatch — e.g. #ifdef __NR_foo guarding #define __SNR_bar — means the
define will silently resolve to __PNR_bar (not present) even when __NR_foo
is defined by the kernel headers. The existing name-set diff (using sort -u)
would not catch this because both names appear in the header; only the pairing
is wrong.

Fixes: Github Issue #315

Fix

Add check_snr_ifdef() to src/arch-syscall-check. The function reads
seccomp-syscalls.h line by line and reports any #ifdef __NR_<X> /
#define __SNR_<Y> pairing where X ≠ Y, along with the file and line number.

Any other preprocessor directive between an #ifdef and the expected
#define (including unconditional defines that carry no #ifdef guard)
resets the pending guard name so only genuine guarded pairs are checked.

The exit code from check_snr_ifdef() is accumulated into the script's
overall return value alongside the existing check_snr() and check_pnr()
checks, so a guard mismatch fails the arch-syscall-check test that runs
as part of make check.

Testing

• Clean pass on the current seccomp-syscalls.h (all guards match).
• Injecting #ifdef __NR_accept_TYPO in place of #ifdef __NR_accept
  produces: MISMATCH at …seccomp-syscalls.h:313: #ifdef __NR_accept_TYPO but #define __SNR_accept
  and exits 1.
• make check passes (PASS: arch-syscall-check, PASS: regression).