kbuild: Check for 'clang' string in the entire --version output

The build system assumes that the clang version string is found in the
first line of the clang --version output. However, this is not true
for SDLLVM. Hence, look for the string in the entire output, rather
than just the first line.

Change-Id: I2db999e616f4e5ceedb5c0081d6440bbd5c3f52e
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>