Respect target-cpu in RUSTFLAGS

[breezewish]

Partially resolve #241

1. in zigbuild command, target-cpu in the RUSTFLAGS (or local Cargo config files) is parsed and appended to the compiler wrapper.

   For example, RUSTFLAGS="-Ctarget-cpu=x86-64-v3" will cause compiler wrapper to be cargo-zigbuild zig cc -- -g -mcpu=x86_64_v3 ...

2. -Ctarget-features is currently not respected in this PR. However it is parsed well.

3. Add a test using https://github.com/breezewish/x86-instruction-set-analyzer to check whether desired instruction set presents in the compiled binary. This helps us make sure that RUSTFLAGS are correctly passed.

   Note: As we check instruction sets for the entire binary, it is possible that the instruction set (for example AVX2) comes from the Rust part, not the C++ part. I tested manually that, when RUSTFLAGS="-C target_cpu=x86-64-v4" is set, the Rust part will not produce AVX2 instructions for tests/target-cpu. If we discovered AVX2 instruction, it must comes from the C++ part.

   Here is the instruction check result with RUSTFLAGS="-C target_cpu=x86-64-v4" without using this PR:

   Format: Elf Little-endian 64-bit
   Kind: Dynamic
   Architecture: X86_64
   
   Instruction set usage (total 144541):
   X64: 93623 (64.77%)
   INTEL386: 31545 (21.82%)
   INTEL8086: 11774 (8.15%)
   SSE: 2660 (1.84%)
   SSE2: 1547 (1.07%)
   CMOV: 1395 (0.97%)
   MULTIBYTENOP: 1280 (0.89%)
   BMI2: 528 (0.37%)
   INTEL286: 46 (0.03%)
   INTEL186: 31 (0.02%)
   BMI1: 31 (0.02%)
   LZCNT: 30 (0.02%)
   INTEL486: 28 (0.02%)
   AVX: 17 (0.01%)
   PAUSE: 4 (0.00%)
   CPUID: 2 (0.00%)
   

[breezewish]

Label CI-no-fail-fast may be needed in order to verify the newly added test case.

Oh, looks like the newly introduced dependency rustflags required rustc 1.74. Ref dtolnay/rustflags#5

As a tool, a high rustc version requirement might be fine, as users can just install latest rustc and then install this tool.

[breezewish]

CI is now passed, except for zig-master branch :)

[messense]

Oh, looks like the newly introduced dependency rustflags required rustc 1.74. Ref dtolnay/rustflags#5

Not a big issue, I'll cut a new release from main before merging this.

[messense]

Mostly LGTM, just a minor nit.