package/uclibc: define BR2_UCLIBC_TARGET_ARCH for ARM64

So far, BR2_UCLIBC_TARGET_ARCH had a value for all
architectures... except for ARM64. Its value is passed as ARCH= when
building uClibc... which is currently empty:

/usr/bin/make -j33 -C .../build/uclibc-1.0.41 ARCH="" CROSS_COMPILE=".../host/bin/aarch64-buildroot-linux-uclibc-"

And also in uclibc.mk, when we inject the architecture in uClibc's
.config:

        $(call KCONFIG_ENABLE_OPT,TARGET_$(UCLIBC_TARGET_ARCH))
        $(call KCONFIG_SET_OPT,TARGET_ARCH,"$(UCLIBC_TARGET_ARCH)")

We enable TARGET_ (which doesn't exist) and set TARGET_ARCH to the
empty string.

Nobody noticed so far, and it happens to work because aarch64 is the
first architecture in the choice of architectures in
extra/Configs/Config.in in the uClibc code. So, when the architecture
is not specified, aarch64 is used.

As we are about to add some logic in uclibc.mk that will use
BR2_UCLIBC_TARGET_ARCH, we noticed this missing definition.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 8195e85e10c30b2284613200b4cb1dcba31c9386)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/uclibc/Config.in

@@ -124,6 +124,7 @@ config BR2_UCLIBC_INSTALL_UTILS
 # uClibc architecture names.
 config BR2_UCLIBC_TARGET_ARCH
 	string
+	default "aarch64"  if BR2_aarch64 || BR2_aarch64_be
 	default "arc"	   if BR2_arcle || BR2_arceb
 	default "arm"	   if BR2_arm	|| BR2_armeb
 	default "m68k"	   if BR2_m68k