optee: enable clang support

Patch 0002 and 0003 in optee-test remove -W options from make
to enable successful compilation with clang.

Patch 0007 modifies the libgcc$(sm) variable in optee so that it can
find the compiler-rt dependencies. Compiler-rt is also added as a
dependency to the optee recipe.

The compiler in use is passed to make via TOOLCHAIN. If TOOLCHAIN
doesn't exist (it is declared by meta-clang, and so doesn't exist
by default), 'gcc' is passed as a default.

These modifications allow optee to be successfully compiled
when TOOLCHAIN=clang

Change-Id: If565969e169078fabd5e3cf5716ca317cc49a126
Signed-off-by: Brett Warren <brett.warren@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>