Use .section .rodata instead of .rodata on FreeBSD

In commit 0a5b942d4 the FreeBSD SECTION_STATIC macro was set to
".rodata". This assembler directive is supported by LLVM (as a
convenience alias for ".section .rodata") by not by GNU as.

This caused the FreeBSD builds that are done with gcc to fail.
Therefore, use ".section .rodata" instead, similar to the other
asm_linkage.h headers.

Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Attila Fülöp <attila@fueloep.org>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Dimitry Andric <dimitry@andric.com>
Closes #14526