Fix kernel toolchain action inputs

[fionera]

Summary

• pass arch/x86/include/asm/inat.h to the x86 instruction-table generator and declare it as an action input
• recover the unique Clang resource include tree from cc_toolchain.all_files when feature-generated flags omit it
• apply the resource-header fallback to both regular and grouped Linux compile actions
• add focused unit coverage for fallback, preservation, and path recognition

Root cause

The x86 generator binary requires -inat_h, but the bzImage action only supplied the opcode map. Separately, Linux compile actions add -nostdinc and previously relied on the configured feature flags to carry Clang's resource include directory; some source-built hermetic LLVM toolchains declare that tree in all_files without emitting the corresponding flag.

Fixes #78.
Fixes #79.

Validation

• focused compile-flag, grouped-action, helper, and buildifier tests: 5/5 passed
• //:kernel_outputs_x86_64_build_test: passed after building the complete x86 image graph (4,474 actions)
• //:kvm_arm64_neon_kernel_build_test: passed after building the ARM64 KVM/NEON kernel graph (3,028 graph actions)
• path-mapped aquery confirms LinuxX86InsnAttr declares both inat.h and the opcode map and passes -inat_h

The broad local //... run reached 151 passing tests with no test failures, but Bazel 9.1 twice crashed internally in Skyframe under the CI state-discard options. A subsequent run without those options exhausted the shared workspace disk while sandboxing unrelated Go links. The draft PR CI is left to run the complete clean matrix.