config: Sync largefile.m4 from binutils-gdb

config: Sync largefile.m4 from binutils-gdb

The following patch improves handling of largefile support with procfs
on 32-bit Solaris.  It has already been approved and installed for
binutils-gdb in the thread starting at

	[PATCH] Unify Solaris procfs and largefile handling
        https://sourceware.org/pipermail/gdb-patches/2020-June/169977.html

I'm syncing the config/largefile.m4 part to gcc now which is the master
for config.  Since ACX_LARGEFILE isn't used anywhere in the gcc tree,
I'm installing it as obvious.

2020-09-09  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	config:
	* largefile.m4: Sync from binutils-gdb.