Feature request: gas .prefalign directive for body-size-dependent function alignment

[Fangrui Song]

Hi, I've filed a feature request
(https://sourceware.org/bugzilla/show_bug.cgi?id=33943 ) proposing a
new directive, `.prefalign`, for size-proportional function alignment.
(LLVM's integrated assembler has already implemented a variant of this
for the `-ffunction-sections` case, though its section
sh_addralign-only semantics felt surprising to me, so I refined the
design.)

## Proposed Syntax

  .prefalign <pref_align>, <end_sym>, nop
  .prefalign <pref_align>, <end_sym>, <fill_byte>

- `pref_align`: preferred (maximum) alignment — must be a power of 2
- `end_sym`: a symbol marking the end of the code body (forward reference)
- Third operand: `nop` for target-appropriate variable-size NOP fill,
or an integer fill byte [0, 255].
  Expressions are not allowed.

All three operands are required. Traditional `.align` implicitly uses
NOPs in executable sections; `.prefalign` makes the fill intent
explicit to avoid that ambiguity.

There is a very minor code-size benefit. For example, both GCC and
Clang prefer .p2align 4 for x86, which is wasteful for small
functions.
The assembler directive, once implemented, would be superior to GCC's
-flimit-function-alignment.