Enable clang compilation with libc but not linker.

Clang is still disabled for x86 and x86_64 long double code,
for x86_64 special assembly instruction, and the linker module.

BUG: 17163651
BUG: 17302991
BUG: 17403674
Change-Id: I43c5f5f0ddc0c2a31918f00b57150dadd8f3f26b