x86: reserve bl and bh registers to match rbx

[CathalMullan]

bl and bh need to be explicitly marked as reserved, as they are sub-registers of rbx, which is reserved by LLVM.

Discovered this while trying to run Graviola through Cranelift, which was becoming corrupted due to the register allocator assigning bl: rust-lang/rustc_codegen_cranelift#1629

cc: @bjorn3

[rustbot]

r? @petrochenkov

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[petrochenkov]

r? @Amanieu