bindgen-cli: unset M-A

grep 'bindgen:any' /var/lib/apt/lists/*_{Sources,Packages} returns nothing, so M-A: allowed for bindgen is currently unused. Also, in reference to https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=749826;filename=multiarch.patch;msg=76,

Whilst binary executables generally differ with architectures, the exposed interface (for example the command line interface, the content on standard input/output, the way they process files) may be independent of the architecture used to execute. In that case, their interface may be considered architecture-independent.

the way bindgen processes files is architecture-dependent. For instance, generating bindings for /usr/include/zlib.h with bindgen:amd64 and bindgen:arm64 (on amd64) returns very different results. Hence bindgen should probably remain M-A: no permanently.