kbuild: clang: Disable sometimes-uninitialized warnings

Disable sometimes-uninitialized warnings as false
positives are reported.

msm-4.14/arch/arm64/include/asm/percpu.h:102:1: warning: variable
'ret' is used uninitialized whenever switch default is taken
[-Wsometimes-uninitialized] error, forbidden warning: percpu.h:102

Change-Id: I5e6b39fba39bf47ffe8451a3d5116175c17aac1a
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>