PR ld/20828: Work around RISC-V failures

Complement commit 81ff47b3 ("PR ld/20828: Fix linker script symbols
wrongly forced local with section GC") and add `.plt' to the list of
output sections created, fixing:

FAIL: PR ld/20828 dynamic symbols with section GC (auxiliary shared library)
FAIL: PR ld/20828 dynamic symbols with section GC (plain)
FAIL: PR ld/20828 dynamic symbols with section GC (version script)

failures with `riscv32-elf', `riscv32-linux', `riscv64-elf' and
`riscv64-linux' targets caused by LD crashing in the absence of such a
section.

	ld/
	PR ld/20828
	* testsuite/ld-elf/pr20828.ld: Add `.plt'.