fix(build): don't rely on that gcc-ar is in the same directory as gcc
ccache - a fast C/C++ compiler cache.
ccache wraps gcc and g++ compilers, but not other tools like ar.
If ccache is installed, then build fails with
```
make: /usr/lib/ccache/aarch64-linux-gnu-gcc-ar: No such file or directory
```
Change-Id: I555c178aeaa0cf411cdf67b4a81e7952be762d0f
Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/make_helpers/toolchain.mk b/make_helpers/toolchain.mk
index 7255509..def6653 100644
--- a/make_helpers/toolchain.mk
+++ b/make_helpers/toolchain.mk
@@ -290,7 +290,7 @@
guess-gnu-gcc-ld = $(if $(filter 1,$(ENABLE_LTO)),$(1),$(shell $(1) --print-prog-name ld.bfd 2>$(nul)))
guess-gnu-gcc-oc = $(shell $(1) --print-prog-name objcopy 2>$(nul))
guess-gnu-gcc-od = $(shell $(1) --print-prog-name objdump 2>$(nul))
-guess-gnu-gcc-ar = $(patsubst %$(notdir $(1)),%$(subst gcc,gcc-ar,$(notdir $(1))),$(1))
+guess-gnu-gcc-ar = $(call which,$(patsubst %$(notdir $(1)),%$(subst gcc,gcc-ar,$(notdir $(1))),$(1)))
define locate-toolchain-tool-cc
$(eval toolchain := $(1))