gallium: Provide alternative stdint.h and stdbool.h C99 headers, instead of ad-hoc definitions.
Everybody is using the C99's integer types. Everybody except Microsoft, which in turns means everybody is including their own definitions of C99 integer types for MSVC, causing duplicate definitions when linking two projects. This is the case of building Gallium and LLVM with MSVC. Shipping alternative stdint.h and stdbool.h headers for MSVC allows us to share a single definition. It also removes clutter from the Gallium headers.