cmd/link: set SizeOfRawData rather than VirtualSize in COFF files for .bss section

GCC and Clang both set the SizeOfRawData field rather than the
VirtualSize field for communicating the size of the .bss section. As a
consequence, LLD does not look at VirtualSize and collapses the .bss
section into whatever is around it, resulting in runtime crashes. This
commit changes the logic so that if the requested "file size" is 0, then
the SizeOfRawData field is set rather than the VirtualSize field as the
sole length marker.

Fixes #44250.
Fixes #39326.
Updates #38755.
Updates #36439.
Updates #43800.

Change-Id: Ied89ddaa0a717fed840238244c6e4848845aeeb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/291630


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>