[Driver] Don't default to -mrelax-all for non-RISCV -O0

Some assembly mnemonics may assemble to instructions of different
lengths. The longer form is to support instructions like a long branch.
On X86, -mrelax-all enables `MCRelaxAll`, which expands instructions to
the long form regardless of whether a short form suffices, while
-mno-relax-all only expands instructions when needed.

```
// x86 example
void foo(int a) {
  // -mno-relax-all or gas: short jump (2 bytes)
  // -mrelax-all: near jump (6 bytes)
  if (a) bar();
}
```

The -mrelax-all default for non-RISCV -O0 appears to only affect x86 and
increases code size without any compile time difference for a stage-2
x86-64 build of lld.
```
-mrelax-all:    file size: 60.9MiB   VM size: 52.4MiB
-mno-relax-all: file size: 58.2MiB   VM size: 49.7MiB
```

There is no compile time difference (other than noise) GNU assembler
doesn't expand instructions by default. Let's remove the -mrelax-all default.

Pull Request: https://github.com/llvm/llvm-project/pull/90013