add -femit-llvm-bc CLI option and implement it, and improve -fcompiler-rt support

[andrewrk]

Emitting LLVM bitcode & improvements to stage2 LLVM backend

• Added doc comments for std.Target.ObjectFormat enum
• std.Target.oFileExt is removed because it is incorrect for Plan-9
  targets. Instead, use std.Target.ObjectFormat.fileExt and pass a
  CPU architecture.
• Added Compilation.Directory.joinZ for when a null byte is desired.
• Improvements to Compilation.create logic for computing use_llvm
  and reporting errors in contradictory flags. -femit-llvm-ir and
  -femit-llvm-bc will now imply -fLLVM.
• Fix compilation when passing .bc files on the command line.
• Improvements to the stage2 LLVM backend:
  • cleaned up error messages and error reporting. Properly bubble up
    some errors rather than dumping to stderr; others turn into panics.
  • properly call ZigLLVMCreateTargetMachine and
    ZigLLVMTargetMachineEmitToFile and implement calculation of the
    respective parameters (cpu features, code model, abi name, lto,
    tsan, etc).
  • LLVM module verification only runs in debug builds of the compiler
  • use LLVMDumpModule rather than printToString because in the case
    that we incorrectly pass a null pointer to LLVM it may crash during
    dumping the module and having it partially printed is helpful in
    this case.
  • support -femit-asm, -fno-emit-bin, -femit-llvm-ir, -femit-llvm-bc
  • Support LLVM backend when used with Mach-O and WASM linkers.

support -fcompiler-rt in conjunction with build-obj

When using build-exe or build-lib -dynamic, -fcompiler-rt means building
compiler-rt into a static library and then linking it into the executable.

When using build-lib, -fcompiler-rt means building compiler-rt into an
object file and then adding it into the static archive.

Before this commit, when using build-obj, zig would build compiler-rt
into an object file, and then on ELF, use lld -r to merge it into the
main object file. Other linker backends of LLD do not support -r to
merge objects, so this failed with error messages for those targets.

Now, -fcompiler-rt when used with build-obj acts as if the user puts
_ = @import("compiler_rt"); inside their root source file. The symbols
of compiler-rt go into the same compilation unit as the root source file.

This is hooked up for stage1 only for now. Once stage2 is capable of
building compiler-rt, it should be hooked up there as well.