Mark C globals with assembly reference to small code model

By default, all globals in C/C++ compiled by clang are allocated
in non-large data sections. See [1] for background on code models.
For PIC (Position independent code), this is fine as long as binary is
small but as binary size increases, users maybe want to use medium/large
code models (-mcmodel=medium) which moves data in to large sections.
As data in these large sections cannot be accessed using PIC code
anymore (as it's too far away), compiler ends up using a different
instruction sequence when building C/C++ code -- using GOT to access
these globals (which can be relaxed by linker at link time if binary
ends up being small). However, assembly files continue to access these
globals defined in C/C++ files using older (and invalid instruction
sequence). So, we mark all such globals with an attribute that forces
them to be allocated in small sections allowing them to validly be
accessed from the assembly code.

This patch should not have any affect on builds that use small code
model, which is the default mode.

[1] https://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models

There is similar patch in dav1d which inspired this patch:
videolan/dav1d!1785