Add PARISC, LOONGARCH64, M68K, SH, and SHEB seccomp architectures

[hayama17]

What type of PR is this?

/kind feature

What this PR does / why we need it

Add missing Arch enum variants for seccomp architecture filtering.

The following architectures are defined in the OCI runtime spec but were absent from the Rust implementation:

OCI string	seccomp.h.in	linux/audit.h	linux/elf-em.h	value
SCMP_ARCH_PARISC	AUDIT_ARCH_PARISC	(EM_PARISC)	EM_PARISC=15	0x0000000f
SCMP_ARCH_PARISC64	AUDIT_ARCH_PARISC64	(EM_PARISC|__AUDIT_ARCH_64BIT)	EM_PARISC=15	0x8000000f
SCMP_ARCH_LOONGARCH64	AUDIT_ARCH_LOONGARCH64	(EM_LOONGARCH|__AUDIT_ARCH_64BIT|__AUDIT_ARCH_LE)	EM_LOONGARCH=258	0xc0000102
SCMP_ARCH_M68K	AUDIT_ARCH_M68K	(EM_68K)	EM_68K=4	0x00000004
SCMP_ARCH_SH	AUDIT_ARCH_SHEL	(EM_SH|__AUDIT_ARCH_LE)	EM_SH=42	0x4000002a
SCMP_ARCH_SHEB	AUDIT_ARCH_SH	(EM_SH)	EM_SH=42	0x0000002a

Flags are defined in linux/audit.h L393-L394:

• __AUDIT_ARCH_64BIT = 0x80000000
• __AUDIT_ARCH_LE = 0x40000000

Note: SCMP_ARCH_SH and SCMP_ARCH_SHEB are intentionally reversed relative to their names — SCMP_ARCH_SH maps to AUDIT_ARCH_SHEL (LE) and SCMP_ARCH_SHEB maps to AUDIT_ARCH_SH (BE), following seccomp.h.in L241-L242.

Which issue(s) this PR fixes

Fixes #311

Special notes for your reviewer

Round-trip tests (to_string / parse) for all new variants are added in src/runtime/linux.rs.

Does this PR introduce a user-facing change?

Add missing seccomp Arch enum variants: PARISC, PARISC64, LOONGARCH64, M68K, SH, SHEB