Set hidden visibility on global symbols accessed from AArch64

From Martin Storsjö: https://patchwork.ffmpeg.org/project/ffmpeg/patch/20220714195325.297691-1-martin@martin.st/

"The AArch64 assembly accesses those symbols directly, without indirection via e.g. the GOT on ELF. In order for this not to require text relocations, those symbols need to be resolved fully at link time, i.e. those symbols can't be interposable.

Normally, so far, this is achieved when linking shared libraries in two ways; we have a version script (libavcodec/libavcodec.v) which marks all symbols that don't start with av* as local. Additionally, we try to add -Wl,-Bsymbolic to the linker options if supported, making sure that such symbol references are resolved fully at link time, instead of making them interposable.

When the libavcodec static library is linked into another shared library, there's no guarantee that it uses similar options (even though that would be favourable), which would end up requiring text relocations in the AArch64 assembly.

Explicitly mark the symbols that are accessed from AArch64 assembly as hidden, so that they are resolved fully at link time even without the version script and -Wl,-Bsymbolic."