Created static libc PIC/no-SSP library to be used by rtld.

Rtld is not compatible with SSP, and since we link libc_pic.a to rtld
to have the basic support like memory and string copy functions, we
have to both carefully limit libc use, and to provide the ssp support
shims. This change makes the libc use in rtld more straighforward but
still limited, and allows to remove the shims, to be done in the next
commit.

Submitted by: Luis Pires
Reviewed by: bdrewery, brooks
Differential revision: https://reviews.freebsd.org/D15283