Use rustc target metadata for build-manifest target lists
This ensures that as long as the target metadata is updated to reflect the current state (tier 3 vs 1/2, host toolchain support), the manifest will also be updated. This avoids an extremely common 'oops' when we move targets between tiers.
This mostly removes a bunch of tier 3 targets from manifests. I didn't spot check against produced artifacts but given that they are tier 3 it's probably reasonable to leave it as-is and bug reporters can fix the target metadata in rustc (or we can stop producing artifacts).
Follow-up work would ideally lint the artifact list as part of nightly/beta/stable builds as well, but for now this is simple and hopefully removes at common source of extra PRs when modifying targets.
Manifest list delta:
```diff
--- old
+++ new
-host:i686-apple-darwin
-host:mips-unknown-linux-gnu
-host:mips64-unknown-linux-gnuabi64
-host:mips64el-unknown-linux-gnuabi64
-host:mipsel-unknown-linux-gnu
-host:mipsisa32r6-unknown-linux-gnu
-host:mipsisa32r6el-unknown-linux-gnu
-host:mipsisa64r6-unknown-linux-gnuabi64
-host:mipsisa64r6el-unknown-linux-gnuabi64
-target:aarch64-unknown-hermit
-target:aarch64-unknown-managarm-mlibc
-target:aarch64-unknown-redox
-target:amdgcn-amd-amdhsa
-target:arm64e-apple-darwin
-target:arm64e-apple-ios
-target:arm64e-apple-tvos
-target:armebv7r-none-eabi
-target:armebv7r-none-eabihf
-target:armv7s-apple-ios
-target:bpfeb-unknown-none
-target:bpfel-unknown-none
-target:csky-unknown-linux-gnuabiv2
-target:csky-unknown-linux-gnuabiv2hf
-target:i386-apple-ios
-target:i586-unknown-redox
-target:i686-apple-darwin
-target:loongarch32-unknown-none
-target:loongarch32-unknown-none-softfloat
-target:m68k-unknown-linux-gnu
-target:m68k-unknown-none-elf
-target:mips-mti-none-elf
-target:mips-unknown-linux-gnu
-target:mips-unknown-linux-musl
-target:mips64-unknown-linux-gnuabi64
-target:mips64-unknown-linux-muslabi64
-target:mips64el-unknown-linux-gnuabi64
-target:mips64el-unknown-linux-muslabi64
-target:mipsel-mti-none-elf
-target:mipsel-unknown-linux-gnu
-target:mipsel-unknown-linux-musl
-target:mipsisa32r6-unknown-linux-gnu
-target:mipsisa32r6el-unknown-linux-gnu
-target:mipsisa64r6-unknown-linux-gnuabi64
-target:mipsisa64r6el-unknown-linux-gnuabi64
-target:riscv32gc-unknown-linux-gnu
-target:riscv32im-risc0-zkvm-elf
-target:riscv32ima-unknown-none-elf
-target:riscv64gc-unknown-hermit
-target:riscv64gc-unknown-linux-musl
-target:riscv64gc-unknown-managarm-mlibc
-target:sparc-unknown-none-elf
-target:x86_64-unikraft-linux-musl
-target:x86_64-unknown-hermit
-target:x86_64-unknown-managarm-mlibc
```
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