toolchain: Print CC command and output when CC invocation fails

We are gobbling up the `$(CC_$(arch))` stderr when testing the
toolchain. This change makes it so we print the command we tried to
invoke and the output from the command.

```
toolchain.mk:183: The coreboot toolchain for 'x86_32' architecture was not found.
toolchain.mk:183: /build/guybrush/tmp/portage/sys-boot/coreboot-9999/files/reclient/ccache /build/guybrush/tmp/portage/sys-boot/coreboot-9999/work/coreboot-sdk/bin/i386-elf-gcc -v
I AM STDERR
toolchain.mk:183: I AM STDOUT
toolchain.mk:219:
toolchain.mk:220: Path to your toolchain is currently set to '/build/guybrush/tmp/portage/sys-boot/coreboot-9999/work/coreboot-sdk/bin'
toolchain.mk:222:
toolchain.mk:223: To build the entire coreboot toolchain: run 'make crossgcc'

```

BUG=b:392874252, b:389737339
TEST=USE_REMOTEEXEC=true BOARD=brya bazel run @portage//internal/packages/stage2/target/board/chromiumos/sys-boot/coreboot:9999_debug

Change-Id: I7c7352c7254c21deb3e4a03106b841ec9f111ba4
Signed-off-by: Raul E Rangel <[email protected]>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86220
Tested-by: build bot (Jenkins) <[email protected]>
Reviewed-by: Jon Murphy <[email protected]>

Author: Raul E Rangel

toolchain.mk

@@ -187,7 +187,10 @@ $(foreach arch,$(sort $(foreach stage,\
 		echo not-coreboot; else echo not-coreboot; fi), \
 		$(eval COMPILERFAIL:=1)\
 		$(warning The coreboot toolchain for '$(arch)'\
-			architecture was not found.)))
+			architecture was not found.)\
+		$(if $(CC_$(arch)),\
+			$(warning $(CC_$(arch)) -v)\
+			$(warning $(shell $(CC_$(arch)) -v)))))
 # If iasl doesn't match the current coreboot version, fail the test
 # TODO: Figure out if iasl is even needed for the build.
 $(if $(shell if [ -n "$(IASL)" ]; then \