[MC,CodeGen] Update .prefalign for symbol-based preferred alignment (#184032)

https://discourse.llvm.org/t/rfc-enhancing-function-alignment-attributes/88019/17
The recently-introduced .prefalign only worked when each function was in
its own section (-ffunction-sections), because the section size gave the
function body size needed for the alignment rule.

This led to -ffunction-sections and -fno-function-sections AsmPrinter
differences (#155529), which is rather unusual.

This patch fixes this AsmPrinter difference by extending .prefalign to
accept an end symbol and a required fill operand:

    .prefalign <log2_align>, <end_sym>, nop
    .prefalign <log2_align>, <end_sym>, <fill_byte>

The first operand is a log2 alignment value (e.g. 4 means 16-byte
alignment). The body size (end_sym_offset - start_offset) determines the
alignment:

    body_size < pref_align   => ComputedAlign = std::bit_ceil(body_size)
    body_size >= pref_align  => ComputedAlign = pref_align

To also enforce a minimum alignment, emit a .p2align before .prefalign.

The fill operand is required: `nop` generates target-appropriate NOP
instructions via writeNopData, while an integer in [0,255] fills the
padding with that byte value.

Initialize MCSection::CurFragList to nullptr and add a null check
to skip ELFObjectWriter-created sections like .strtab/.symtab
that never receive changeSection calls.

relaxPrefAlign is called in both layoutSection and relaxFragment.
The layoutSection call ensures correct initial padding before
relaxOnce, and is also needed for the post-finishLayout re-layout
where relaxOnce is not used. relaxPrefAlign walks forward to the
end symbol to compute BodySize (summing fragment sizes), avoiding
dependence on stale downstream symbol offsets.