glslang: update to 16.4.0; shaderc: update to 2026.3.

[Rutpiv]

Testing the changes

• I tested the changes in this PR: briefly

Local build testing

• I built this PR locally for my native architecture, (x86_64-glibc)
• I built this PR locally for these architectures using specific masterdirs:
  • x86_64-musl
  • i686
• I built this PR locally for these architectures (crossbuilds):
  • aarch64
  • aarch64-musl
  • armv7l
  • armv7l-musl
  • armv6l
  • armv6l-musl

---

Besides the version bumps, both templates carry cleanup found while reviewing the build logs.

glslang: the devel dep now uses >=, the exact version made the 32bit runtime deps hook print a parse error. Removed bison (grammar ships pregenerated) and gtest-devel (only reached via ALLOW_EXTERNAL_GTEST, whose golden output tests fail against system SPIRV-Tools). Declared the full license composition and installed upstream LICENSE.txt via vlicense instead of extracting one header from CodeGen.cpp. Switched Release to None with explicit -DNDEBUG to honor xbps CFLAGS, upstream defaults to Debug when unset. Updated the stale header comment.

shaderc: switched to vsed, replaced the dead examples sed with -DSHADERC_SKIP_EXAMPLES=ON and added -DSHADERC_SKIP_COPYRIGHT_CHECK=ON to drop the upstream lint target.