go-pathrs: fix build failure with Clang

[AkihiroSuda]

The PATHRS_PROC_* values are defined in <pathrs.h> as an enum with values that are too large to fit in an int, and CGo's interpretation of their signedness turns out to be compiler-dependent: GCC hands them to us as implicitly-converted (negative) signed constants, while Clang gives us the actual unsigned values. Our sanity check in init() only handled the GCC behaviour (by assigning them to an int64), so builds using Clang failed with

  libpathrs_linux.go:256:32: cannot use (_Ciconst_PATHRS_PROC_ROOT)
    (untyped int constant 18446744067006164835) as int64 value in variable
    declaration (overflows)

Neither int64 nor uint64 can hold the constant with both compilers. However, Go constant expressions are arbitrary-precision and bitwise operations on negative constants operate on their infinite two's complement representation, so masking off the low 64 bits gives us the same uint64 value regardless of which compiler CGo used.

Note that this only affected the internal cross-check of the values (the constants themselves are open-coded on the Go side), so no user-visible behaviour changes -- but the package could not be built at all with Clang.

Also add a CC={gcc,clang} dimension to the Go smoke-test matrix so this doesn't regress. CC is only set for the smoke-test step so that we don't change how libpathrs itself is built.

Fixes #401

Assisted-by: Claude Opus 5

---

Tested with gcc 15.2.0 and clang 21.1.8 (Ubuntu 26.04, go 1.26.7) locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!