PR27597, nios: assertion fail in nios2_elf32_install_imm16

PR27597, nios: assertion fail in nios2_elf32_install_imm16

The assertion in nios2_elf32_install_imm16 triggers when the PLT is
twice the maximum allowable size for a branch from PLTn to reach
.PLTresolve, and on no other call to nios2_elf32_install_imm16.  That
makes the assertion completely useless.  We can handle a PIC PLT
exceeding 0x8000 in size by bouncing branches that won't reach through
previous branches.

	PR 27597
	* elf32-nios2.c (nios2_elf32_install_imm16): Delete BFD_ASSERT.
	(nios2_build_one_stub): Don't bother masking value passed to
	nios2_elf32_install_imm16.
	(nios2_elf32_finish_dynamic_symbol): Likewise.  Handle overflow
	of PLTn branch to .PLTresolve by bouncing through prior branches.