pcg-c: Add 'static' to inline function definitions

Make the inlines static to avoid kernel build failure with Clang 11 on i386.
(The issue was not observed with Clang 10, currently in tree; reproduction
depends on compiler inlining choices.)

The compiler may choose not to inline 'bare' C inlines, and in that case
expects a symbol of the same name will be available. It does not
automatically define that symbol at use, because of traditional C linking
semantics. (In contrast, C++ does define it, and then deduplicates redundant
definitions at link). As we do not instantiate the C99 inline ('extern
inline ...;'), the linker errors with "undefined symbol."

Reported by: dim
Tested by: dim
Fixes: r364219