cmd/asm: align an instruction or a function's address
Recently, the gVisor project needs an instruction's address
with 128 bytes alignment to fit the architecture requirement
for interrupt table.
This patch allows aligning an instruction's address to be
aligned to a specific value (2^n and in the range [8, 2048])
The main changes include:
1. Adds a new element in the FuncInfo structure defined in
cmd/internal/obj/link.go file to record the alignment
information.
2. Adds a new element in the Func structure defined in
cmd/internal/goobj/read.go file to read the alignment
information.
3. Adds the assembler support to align an intruction's offset
with a specific value (2^n and in the range [8, 2048]).
e.g. "PCALIGN $256" indicates that the next instruction should
be aligned to 256 bytes.
4. An instruction's alignment is relative to the start of the
function where this instruction is located, so the function's
address must be aligned to the same or coarser boundary.