`build`: generate hex dump of server assets during build

[ochafik]

Currently one needs to manually regenerate asset files in examples/server, which:

• is easy to forget
• ~~may cause a security risk~~ (reviewers are unlikely to unescape the code - don't wanna be the next target after xz) (edit: already mitigated by https://github.com/ggerganov/llama.cpp/pull/6409 as pointed out below)

od is used in Makefile (unlike xxd, it should be available on ~all systems, incl. on Windows w/ w64devkit - thanks to busybox), while cmake and zig builds cast their own hex for portability (may have made me n-curse).