Move c_getstr to fold-const.c

Move c_getstr to fold-const.c

Upcoming patches to fold-const-call.c want to use c_getstr, which is
currently defined in builtins.c.  The function doesn't really do anything
related to built-ins, and I'd rather not make fold-const-call.c depend
on builtins.c and builtins.c depend on fold-const-call.c, so this patch
moves the function to fold-const.c instead.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* builtins.h (c_getstr): Move to...
	* fold-const.h (c_getstr): ...here.
	* builtins.c (c_getstr): Move to...
	* fold-const.c (c_getstr): ...here.

From-SVN: r229919