elf: Enable relro for static build

The code is similar to the one at elf/dl-reloc.c, where it checks for
the l_relro_size from the link_map (obtained from PT_GNU_RELRO header
from program headers) and calls_dl_protected_relro.

For testing I will use the ones proposed by Florian's patch
'elf: Add tests for working RELRO protection' [1].

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
aarch64-linux-gnu, s390x-linux-gnu, and sparc64-linux-gnu.  I also
check with --enable-static pie on x86_64-linux-gnu, i686-linux-gnu,
and aarch64-linux-gnu which seems the only architectures where
static PIE is actually working (as per 9d7a3741, on
arm-linux-gnueabihf, powerpc64{le}-linux-gnu, and s390x-linux-gnu
I am seeing runtime issues not related to my patch).

[1] https://sourceware.org/ml/libc-alpha/2019-10/msg00059.html



Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>