RISC-V: check offsets when linker relaxation is disabled

RISC-V: check offsets when linker relaxation is disabled

The assembler partially relied on the linker to check whether the
offset is valid.  However, some optimization logic (added later)
removes relocations relative to local symbols without checking offsets.

For instance, it caused following code to silently emit wrong jumps
(to the jump instruction "." itself) without relocations:

> .option norelax
> j .+0x200000   # J (or JAL) instruction cannot encode this offset.
> j .+1          # Jump to odd address is not valid.

This commit adds offset checks where necessary.

gas/ChangeLog:

	* config/tc-riscv.c (md_apply_fix): Check offsets when the
	relocation relative to a local symbol is being optimized out.
	* testsuite/gas/riscv/no-relax-branch-offset-fail.s: Failure
	case where the branch offset is invalid.
	* testsuite/gas/riscv/no-relax-branch-offset-fail.d: Ditto.
	* testsuite/gas/riscv/no-relax-branch-offset-fail.l: Ditto.
	* testsuite/gas/riscv/no-relax-branch-offset-ok.s: Border case.
	* testsuite/gas/riscv/no-relax-branch-offset-ok.d: Ditto.
	* testsuite/gas/riscv/no-relax-pcrel-offset-fail-64.s: Failure
	case only on RV64 where the PC-relative offset exceed limits.
	* testsuite/gas/riscv/no-relax-pcrel-offset-fail-64.d: Ditto.
	* testsuite/gas/riscv/no-relax-pcrel-offset-fail-64.l: Ditto.
	* testsuite/gas/riscv/no-relax-pcrel-offset-fail-not-32.d: Test
	case for RV32 so that no errors occur.
	* testsuite/gas/riscv/no-relax-pcrel-offset-ok.s: Border case.
	* testsuite/gas/riscv/no-relax-pcrel-offset-ok.d: Ditto.