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