core: fix setting of HOSTARCH

Currently, we set HOSTARCH to the output of `uname -m`. This gives us
the architecture as seen by the running kernel. For example, we would
end up with 'x86_64' for a 64-bit kernel running on an x86_64 processor.

We use that value to determine whether we can run some binary tools,
like our pre-configured external toolchains.

However, one may be running a userland in a different bitness than that
of the running kernel. For example, one may run in a 32-bit chroot, even
though the kernel is running in 64-bit.

Up until recently, this was not an issue because the pre-configured
external toolchains were all requiring an i386 (x86 in Buildroot
parlance).

But since we introduced the latest Linaro toolchains, we now have
toolchains that require a 64-bit userland.

So, when running on a 64-bit kernel, we believe those toolchains are
available, even when the user is running a 32-bit userland. This causes
build failures for our autobuilders, like so:

    http://autobuild.buildroot.org/results/9cd/9cdf10ec5b31144b2e03ea09cf128702339895b3/

with the following symptoms:

    >>> toolchain-external undefined Configuring
    Cannot execute cross-compiler '/home/test/autobuild/instance-3/output/host/opt/ext-toolchain/bin/aarch64-linux-gnu-gcc'

So, instead of relying on the output of `uname -r`, look for the host
gcc and extract the target it was configured to generate code for.

Fixes:
    http://autobuild.buildroot.org/results/9cd/9cdf10ec5b31144b2e03ea09cf128702339895b3/  (aarch64)
    http://autobuild.buildroot.org/results/888/8889aa7d9fb48370e4760a6edbc6d3ae945f02f2/  (arm)
    and many more...

Besides fixing those issues, it will also allow us to add the 64-bit
variants of toolchains when they exist, like the upcoming Codescape
MTI and IMG toolchains for MIPS from Imagination Technologies.

[Peter: use HOSTCC_NOCCACHE]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -52,16 +52,6 @@ ifneq ($(firstword $(sort $(RUNNING_MAKE_VERSION) $(MIN_MAKE_VERSION))),$(MIN_MA
 $(error You have make '$(RUNNING_MAKE_VERSION)' installed. GNU make >= $(MIN_MAKE_VERSION) is required)
 endif
 
-export HOSTARCH := $(shell uname -m | \
-	sed -e s/i.86/x86/ \
-	    -e s/sun4u/sparc64/ \
-	    -e s/arm.*/arm/ \
-	    -e s/sa110/arm/ \
-	    -e s/ppc64/powerpc64/ \
-	    -e s/ppc/powerpc/ \
-	    -e s/macppc/powerpc/\
-	    -e s/sh.*/sh/)
-
 # Parallel execution of this Makefile is disabled because it changes
 # the packages building order, that can be a problem for two reasons:
 # - If a package has an unspecified optional dependency and that
@@ -293,6 +283,32 @@ HOSTRANLIB := $(shell which $(HOSTRANLIB) || type -p $(HOSTRANLIB) || echo ranli
 export HOSTAR HOSTAS HOSTCC HOSTCXX HOSTLD
 export HOSTCC_NOCCACHE HOSTCXX_NOCCACHE
 
+# Determine the userland we are running on.
+#
+# Note that, despite its name, we are not interested in the actual
+# architecture name. This is mostly used to determine whether some
+# of the binary tools (e.g. pre-built external toolchains) can run
+# on the current host. So we need to know if the userland we're
+# running on can actually run those toolchains.
+#
+# For example, a 64-bit prebuilt toolchain will not run on a 64-bit
+# kernel if the userland is 32-bit (e.g. in a chroot for example).
+#
+# So, we extract the first part of the tuple the host gcc was
+# configured to generate code for; we assume this is our userland.
+#
+export HOSTARCH := $(shell $(HOSTCC_NOCCACHE) -v 2>&1 | \
+	sed -e '/^Target: \([^-]*\).*/!d' \
+	    -e 's//\1/' \
+	    -e 's/i.86/x86/' \
+	    -e 's/sun4u/sparc64/' \
+	    -e 's/arm.*/arm/' \
+	    -e 's/sa110/arm/' \
+	    -e 's/ppc64/powerpc64/' \
+	    -e 's/ppc/powerpc/' \
+	    -e 's/macppc/powerpc/' \
+	    -e 's/sh.*/sh/' )
+
 HOSTCC_VERSION := $(shell $(HOSTCC_NOCCACHE) --version | \
 	sed -n -r 's/^.* ([0-9]*)\.([0-9]*)\.([0-9]*)[ ]*.*/\1 \2/p')