Use fewer instructions for `fail!`
Adds a special-case fail function, rustrt::unwind::begin_unwind_no_time_to_explain,
that encapsulates the printing of the words "explicit failure".
The before/after optimized assembly:
```
leaq "str\"str\"(1369)"(%rip), %rax
movq %rax, 8(%rsp)
movq $19, 16(%rsp)
leaq 8(%rsp), %rdi
movl $11, %esi
callq _ZN6unwind31begin_unwind_no_time_to_explain20hd1c720cdde6a116480dE@PLT
```
```
leaq "str\"str\"(1412)"(%rip), %rax
movq %rax, 24(%rsp)
movq $16, 32(%rsp)
leaq "str\"str\"(1413)"(%rip), %rax
movq %rax, 8(%rsp)
movq $19, 16(%rsp)
leaq 24(%rsp), %rdi
leaq 8(%rsp), %rsi
movl $11, %edx
callq _ZN6unwind12begin_unwind21h15836560661922107792E
```
Before/after filesizes:
rwxrwxr-x 1 brian brian
21479503 Jul 20 22:09 stage2-old/lib/librustc-
4e7c5e5c.so
rwxrwxr-x 1 brian brian
21475415 Jul 20 22:30 x86_64-unknown-linux-gnu/stage2/lib/librustc-
4e7c5e5c.so