Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Mark csu on PowerPC as not requiring executable stack.

Reviewed and tested by: nwhitehorn

Let all .c and .S files under lib/csu consistently use the __FBSDID()
macro for identification, instead of several different hand-rolled
variants (plain .ident, .ascii, etc).

Use .rodata section for $FreeBSD$.

PowerPC verions of the crt initialization and finalization files required
by the ELF ABI.