RISC-V: Error for relaxable branch in absolute section.

RISC-V: Error for relaxable branch in absolute section.

Emit an error instead of crashing in frag_new, handling this same as the
i386 port.

gas/
	PR 26400
	* config/tc-riscv.c (append_insn): If in absolute section, emit
	error before add_relaxed_insn call.
	* testsuite/gas/riscv/absolute-sec.d: New.
	* testsuite/gas/riscv/absolute-sec.l: New.
	* testsuite/gas/riscv/absolute-sec.s: New.