[flang][Driver] Add -masm option to flang (#81490)

The motivation here was a suggestion over in Compiler Explorer. You can
use `-mllvm` already to do this but since gfortran supports `-masm`, I
figured I'd try to add it.

This is done by flang expanding `-masm` into `-mllvm x86-asm-syntax=`,
then passing that to fc1. Which then collects all the `-mllvm` options
and forwards them on.

The code to expand it comes from clang `Clang::AddX86TargetArgs` (there
are some other places doing the same thing too). However I've removed
the `-inline-asm` that clang adds, as fortran doesn't have inline
assembly.

So `-masm` for flang purely changes the style of assembly output.

```
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu
<...>
        pushq   %rbp
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu -masm=att
<...>
        pushq   %rbp
$ ./bin/flang-new /tmp/test.f90 -o - -S -target x86_64-linux-gnu -masm=intel
<...>
        push    rbp
```

The test is adapted from `clang/test/Driver/masm.c` by removing the
clang-cl related lines and changing the 32 bit triples to 64 bit triples
since flang doesn't support 32 bit targets.