BareMetal: detect usr/include/c++/v1 path in sysroot

Currently baremetal driver adds <sysroot>/include/c++/v1
for libc++ headers. However on ChromeOS, all include files
are inside <sysroot>/usr/include. So add
<sysroot>/usr/include/c++/v1 if it exists in baremetal driver.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D134478