[ELF] -r: Synthesize R_RISCV_ALIGN at input section start

Without linker relaxation enabled for a particular relocatable file or
section (e.g., using .option norelax), the assembler will not generate
R_RISCV_ALIGN relocations for alignment directives. This becomes
problematic in a two-stage linking process:

```
ld -r a.o b.o -o ab.o
// b.o is norelax. Its alignment information is lost in ab.o.
ld ab.o -o ab
```

When ab.o is linked into an executable, the preceding relaxed section
(a.o's content) might shrink. Since there's no R_RISCV_ALIGN relocation
in b.o for the linker to act upon, the `.word 0x3a393837` data in b.o
may end up unaligned in the final executable.

To address the issue, this patch inserts NOP bytes and synthesizes an
R_RISCV_ALIGN relocation at the beginning of a text section when the
alignment >= 4.

For simplicity, when RVC is disabled, we synthesize an ALIGN relocation
(addend: 2) for a 4-byte aligned section, allowing the linker to trim
the excess 2 bytes.

See also https://sourceware.org/bugzilla/show_bug.cgi?id=33236

Pull Request: https://github.com/llvm/llvm-project/pull/151639

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