install.texi: Document --enable-decimal-float for AArch64

When I committed the patches to enable support for DFP on AArch64, I
forgot to update the installation documentation.

This patch adds AArch64 as needed (same as i386/x86_64).

2023-04-17  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/
	* doc/install.texi (enable-decimal-float): Add AArch64.