RISC-V: Fix a gp relaxation reloc overflow error.

This was broken when I changed how we compute the value for the gp register.
It used to be computed inside the sdata section.  Now it is computed at the
end which makes it an abs section symbol.  There is code that tries to use
the alignment of the section that the gp value is in, but this does not work
if it is in the abs section, as the abs section has alignment of 1 byte.
There are people using alternative linker scripts that still define it in the
sdata section, so the code is still useful.  Thus adding a check to disable
this when gp is in the abs section.

	bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_lui): Add check to exclude abs
	section when setting max_alignment.  Update comment.
	(_bfd_riscv_relax_pc): Likewise.