cross-build: This is not glibc specific, but Linux specific

Neither glibc nor musl define these interfaces, so provide them for all
Linux builds.

Sponsored by: Netflix
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45355

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

Add missing FreeBSD functions to -legacy when building on macOS/Linux

In most cases this simply builds the file from lib/libc for missing
functions (e.g. strlcpy on Linux etc.). In cases where this is not possible
I've added an implementation to tools/build/cross-build.

The fgetln.c/fgetwln.c/closefrom.c compatibility code was obtained from
https://gitlab.freedesktop.org/libbsd/libbsd, but I'm not sure it makes
sense to import it into to contrib just for these three bootstrap files.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D25978