toolchain: update to Clang 12.0.0

Clang 12.0.0 was released on April 15 2021 (with amd64 binaries uploaded
on April 22). It notably fixes the following llvm-objdump warnings [1]:

 $ make COMPILER=clang optee-os 2>&1 | grep -B 1 objdump
   OBJDUMP out/arm/core/tee.dmp
 llvm-objdump: warning: 'out/arm/core/tee.elf': failed to parse debug information for out/arm/core/tee.elf
 --
   OBJDUMP out/arm/ldelf/ldelf.dmp
 llvm-objdump: warning: 'out/arm/ldelf/ldelf.elf': failed to parse debug information for out/arm/ldelf/ldelf.elf
 --
   OBJDUMP out/arm/ta/avb/023f8f1a-292a-432b-8fc4-de8471358067.dmp
 llvm-objdump: warning: 'out/arm/ta/avb/023f8f1a-292a-432b-8fc4-de8471358067.elf': failed to parse debug information for out/arm/ta/avb/023f8f1a-292a-432b-8fc4-de8471358067.elf
 --
   OBJDUMP out/arm/ta/pkcs11/fd02c9da-306c-48c7-a49c-bbd827ae86ee.dmp
 llvm-objdump: warning: 'out/arm/ta/pkcs11/fd02c9da-306c-48c7-a49c-bbd827ae86ee.elf': failed to parse debug information for out/arm/ta/pkcs11/fd02c9da-306c-48c7-a49c-bbd827ae86ee.elf

Link: [1] https://github.com/OP-TEE/optee_os/issues/3808


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

toolchain.mk

@@ -40,7 +40,7 @@ aarch32:
 aarch64:
 	$(call dltc,$(AARCH64_PATH),$(SRC_AARCH64_GCC),$(AARCH64_GCC_VERSION))
 
-CLANG_VER			?= 9.0.1
+CLANG_VER			?= 12.0.0
 CLANG_PATH			?= $(ROOT)/clang-$(CLANG_VER)
 
 # Download the Clang compiler with LLVM tools and compiler-rt libraries