Add function attribute "patchable-function-prefix" to support -fpatchable-function...

Add function attribute "patchable-function-prefix" to support -fpatchable-function-entry=N,M where M>0

Similar to the function attribute `prefix` (prefix data),
"patchable-function-prefix" inserts data (M NOPs) before the function
entry label.

-fpatchable-function-entry=2,1 (1 NOP before entry, 1 NOP after entry)
will look like:

```
  .type	foo,@function
.Ltmp0:               # @foo
  nop
foo:
.Lfunc_begin0:
  # optional `bti c` (AArch64 Branch Target Identification) or
  # `endbr64` (Intel Indirect Branch Tracking)
  nop

  .section  __patchable_function_entries,"awo",@progbits,get,unique,0
  .p2align  3
  .quad .Ltmp0
```

-fpatchable-function-entry=N,0 + -mbranch-protection=bti/-fcf-protection=branch has two reasonable
placements (https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01185.html):

```
(a)         (b)

func:       func:
.Ltmp0:     bti c
  bti c     .Ltmp0:
  nop       nop
```

(a) needs no additional code. If the consensus is to go for (b), we will
need more code in AArch64BranchTargets.cpp / X86IndirectBranchTracking.cpp .

Differential Revision: https://reviews.llvm.org/D73070