[gold] RFC: Add options for controlling mmap and fallocate of output file

[Cary Coutant]

> I don't understand why you are bothering with <linux/falloc.h> and
> FALLOC_FL_KEEP_SIZE.  Why not just pass 0 as the mode argument to
> fallocate?

Careless reading of the man page :-). For some reason, I read it to
mean I *had* to pass that flag. I'll fix it.

Don't I still need to include <linux/falloc.h> and then call
::fallocate under an #ifdef HAVE_LINUX_FALLOC_H? The only declaration
for fallocate() I found in the Linux headers is in that header file.
Since the man page said it's highly non-portable, I figured I needed
the configure test.

-cary