gallium: Provide alternative stdint.h and stdbool.h C99 headers, instead of ad-hoc...

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.