RISC-V: Check the overflow for %pcrel_lo addend more strictly.

The %pcrel_lo addend may causes the overflow, and need more than one
%pcrel_hi values.  But there may be only one auipc, shared by those
%pcrel_lo with addends.  However, the existing check method in the
riscv_resolve_pcrel_lo_relocs, may not be able to work for some
special/corner cases.

Consider the testcases pcrel-lo-addend-2b.  Before applying this patch,
I can compile it successfully.  But in fact the addend cause the value
of %pcrel_hi to be different.  This patch try to check the value of
%pcrel_hi directly, to make sure it won't be changed.  Otherwise, linker
will report the following errors,

(.text+0xa): dangerous relocation: %pcrel_lo overflow with an addend,
the value of %pcrel_hi is 0x1000 without any addend, but may be 0x2000
after adding the %pcrel_lo addend

The toolchain regressions, rv64gc-linux/rv64gc-elf/rv32gc-linux/rv32i-elf,
pass expectedly and looks fine.

bfd/
    * elfnn-riscv.c (riscv_resolve_pcrel_lo_relocs): Check...