Always read from all CFLAGS-style flags

[madsmtm]

Reading from just one of these means that users setting different environment variables in different parts of the build process would not get flags from the other parts.

This tripped me up in rust-lang/rust#133092 / rust-lang/rust#136984, where I thought that since I was extending CFLAGS_* instead of overwriting it, I was allowing cc-rs to also read from CFLAGS.

Instead, we now read all of the flags, but in order such that more specific flags override less specific ones.

So e.g. when users set CFLAGS=-flag1 CFLAGS_aarch64_apple_darwin=-flag2 cargo build, we end up passing -flag1 -flag2 to the compiler (in that order specifically).

NOTE: This has the slight chance of breaking builds where users assume that CFLAGS_$TARGET overwrites CFLAGS. I will argue that that is likely to be the minority, relative to users that do want flags from all CFLAGS* env vars.
To fix such cases, users should pass the negative flag in CFLAGS_$TARGET that they don't want CFLAGS to set (e.g. if CFLAGS contains -pthread, users should pass CFLAGS_$TARGET=-no-pthread).

[NobodyXu]

Thanks, I think this PR is sound, just have some code maintenance suggestion