MFC r340228-r340229, r340231

MFC r340228-r340229, r340231

r340228 by jhb:
Enable use of a global shared page for RISC-V.

machine/vmparam.h already defines the SHAREDPAGE constant.  This
change just enables it for ELF executables.  The only use of the
shared page currently is to hold the signal trampoline.

Reviewed by:	markj, kib
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D17875

r340229 by jhb:
Drop the legacy ELF brandinfo for the old rtld from arm64 and riscv.

These architectures never shipped binaries with an rtld path of
/usr/libexec/ld-elf.so.1.

Reviewed by:	markj
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D17876

r340231 by jhb:
Enable non-executable stacks by default on RISC-V.

Reviewed by:	markj
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D17878