main/linux-firmware: compress firmware with zstd

Uses the upstream install-zst target. ZSTD_CLEVEL=19 saves around 40 MB over the default (which is 3, I think).

• Uncompressed: 1,0 GB
• ZSTD_CLEVEL unset: 509,1 MB
• ZSTD_CLEVEL=10: 494,4 MB
• ZSTD_CLEVEL=19: 469,7 MB

xz with the default compression level was 445,8 MB, but it is slower to decompress and about as slow as ZSTD_CLEVEL=19 to compress. I didn't measure the exact time, but it felt similarly long. ZSTD_CLEVEL=10 and ZSTD_CLEVEL unset were fast to compress. These tests were conducted on an i5-6200U with 8 GB of RAM, so not the beefiest system.

Closes #15756 (closed)

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TODO:

• postmarketos-mkinitfs support: https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/merge_requests/49
• (Alpine) mkinitfs support: mkinitfs!164 (merged)
• New (Alpine) mkinitfs release: mkinitfs@f95e5508
• New (Alpine) mkinitfs release merged in Alpine: b214163a
• pd-mapper support, pd-mapper moved into kernel, or ".jsn" files marked as uncompressable upstream: All ".jsn" files are marked as RawFile in WHENCE: https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/WHENCE
• tqftpserv support, or relevant firmware marked as uncompressable upstream: https://github.com/linux-msm/tqftpserv/pull/20
• New tqftpserv release: https://github.com/linux-msm/tqftpserv/releases/tag/v1.1
• New tqftpserv release merged in pmOS: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/5370
• CONFIG_FW_LOADER_COMPRESS_ZSTD enabled in linux-lts: 8ff17746
• CONFIG_FW_LOADER_COMPRESS_ZSTD enabled in linux-edge: 492db628
• CONFIG_FW_LOADER_COMPRESS_ZSTD enabled in linux-rpi 6e1b700e
• CONFIG_FW_LOADER_COMPRESS_ZSTD enabled in linux-virt 70507108
• CONFIG_FW_LOADER_COMPRESS_ZSTD enabled in linux-openpax 47451f18

More things?