ELF: Synthesize R_RISCV_ALIGN at input section start

[MaskRay]

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