PR c/82922
runtime, syscall: use full prototypes in C code
Based on patch by Martin Sebor.
Reviewed-on: https://go-review.googlesource.com/86815
From-SVN: r256437
Calls to library functions now use entersyscall and
exitsyscall as appropriate. This is a first step toward
multiplexing goroutines onto threads.
From-SVN: r180345