Inline assembly clobber formatting mismatch

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The rust book states that asm! clobbers should be written as "{rdx}" but when you use them as such the clobbers will be silently ignored making for some nasty bugs.

Relevant part of the book

testcase

rustc --version:

rustc 1.11.0-nightly (6dcc2c1de 2016-06-22)

rustc output

$ rustc -g main.rs -C opt-level=2
$

Relevant part of objdump:

   140001070:   48 c7 c2 01 00 00 00    mov    $0x1,%rdx
   140001077:   48 89 f0                mov    %rsi,%rax
   14000107a:   ff d0                   callq  *%rax
   14000107c:   48 89 c1                mov    %rax,%rcx
   14000107f:   48 89 d0                mov    %rdx,%rax

Relevant LLVM IR:

  %13 = tail call i64 asm sideeffect inteldialect "mov rdx, 1", "={rdx},~{dirflag},~{fpsr},~{flags}"() #8, !dbg !718, !srcloc !720
  %14 = tail call i64 asm sideeffect inteldialect "call rax", "={rax},{rax},{rcx},~{{rdx}},~{{memory}},~{{cc}},~{dirflag},~{fpsr},~{flags}"(i8* %2, i8 %12) #8, !dbg !739, !srcloc 
  %15 = tail call i64 asm sideeffect inteldialect "mov rax, rdx", "={rax},{rdx},~{dirflag},~{fpsr},~{flags}"(i64 %13) #8, !dbg !742, !srcloc !744

As we can seen in the emitted LLVM IR, the given clobbers are surrounded by {{}}. I'm not sure what effect this has, but it seems like LLVM just silently ignores them. A fix would either be to not send the extra {}'s to LLVM or to edit the documentation to reflect that clobbers should be given without surrounding {}'s. Either way, this should probably not generate invalid code silently.