llvmGen: Remove -fast-llvm flag

Issue #18076 drew my attention to the undocumented `-fast-llvm` flag for
the LLVM code generator introduced in
22733532. Speaking to Moritz about this,
the motivation for this flag was to avoid potential incompatibilities
between LLVM and the assembler/linker toolchain by making LLVM
responsible for machine-code generation.

Unfortunately, this cannot possibly work: the LLVM backend's mangler
performs a number of transforms on the assembler generated by LLVM that
are necessary for correctness. These are currently:

 * mangling Haskell functions' symbol types to be `object` instead of
   `function` on ELF platforms (necessary for tables-next-to-code)
 * mangling AVX instructions to ensure that we don't assume alignment
   (which LLVM otherwise does)
 * mangling Darwin's  subsections-via-symbols directives

Given that these are all necessary I don't believe that we can support
`-fast-llvm`. Let's rather remove it.