lto: Compile PR ld/23818 test with -fPIC

Compile PR ld/23818 test with -fPIC to fix

testsuite/ld-plugin/pr23818b.c:6:(.text+0x34): relocation R_MIPS_26 against `a local symbol' cannot be used when making a shared object; recompile with -fPIC
FAIL: Build libpr23818.so

for mips.

	PR ld/23818
	* testsuite/ld-plugin/lto.exp: Compile PR ld/23818 test with
	-fPIC.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>