package/arm-gnu-a-toolchain: new package

package/arm-gnu-a-toolchain: new package

A number of AArch64 platforms need to build some bits of ARM32 code,
for example in TF-A (ARM Trusted Firmware) or other
firmware/bootloader.

This package allows to get a pre-built cross-compilation toolchain to
build bare-metal ARM32 code.

https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
[Thomas:
 - drop empty LICENSE_FILES
 - use VERSION variable in SITE variable
 - simplify symlink creation
 - drop visible Config.in.host option]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>