cmd/asm: align an instruction or a function's address

Recently, the gVisor project needs an instruction's address
with 128 bytes alignment and a function's start address with
2K bytes alignment to fit the architecture requirement for
interrupt table.

This patch allows aligning the address of an instruction to be
aligned to a specific value (2^n and not higher than 2048) and
the address of a function to be 2048 bytes.

The main changes include:

1. Adds ALIGN2048 flag to align a function's address with
2048 bytes.
e.g. "TEXT ·Add(SB),NOSPLIT|ALIGN2048" indicates that the
address of Add function should be aligned to 2048 bytes.

2. Adds a new element in the FuncInfo structure defined in
cmd/internal/obj/link.go file to record the alignment
information.

3. Adds a new element in the Func structure defined in
cmd/internal/goobj/read.go file to read the alignment
information.

4. Because go introduces a new object file format, also add
a new element in the FuncInfo structure defined in
cmd/internal/goobj2/funcinfo.go to record the alignment
information.

5. Adds the assembler support to align an intruction's offset
with a specific value (2^n and not higher than 2048).
e.g. "PCALIGN $256" indicates that the next instruction should
be aligned to 256 bytes.

This CL also adds a test.

Change-Id: I31cfa6fb5bc35dee2c44bf65913e90cddfcb492a
Reviewed-on: https://go-review.googlesource.com/c/go/+/212767


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>