Add hook to allowing signing of Hexagon code
Add hook to allowing signing of Hexagon code

This allows a build tool to define an env var (HL_HEXAGON_CODE_SIGNER)
to sign Hexagon code; since Hexagon code is typically embedded as data,
it’s nontrivial to extract/sign/replace the code at a higher level in
the build system.


Former-commit-id: d5ed83b1b4b02beb36f4f5c4eb909f0b3716bee1
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide