gcc: Use ld -r when checking for HAVE_LD_RO_RW_SECTION_MIXING

Fall back to ld -r if ld -shared fails during configure. The check for
HAVE_LD_RO_RW_SECTION_MIXING can fail on targets where ld does not
support shared objects, even though the answer to the test should be
'read-write'. One such target is riscv64-unknown-elf. Failing this test
results in a libgcc crtbegin.o which has a writable .eh_frame section
leading to the default linker scripts placing the .eh_frame section in a
writable memory segment, or a linker warning when using ld scripts that
place .eh_frame unconditionally in ROM.

gcc/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Use ld -r in the check for HAVE_LD_RO_RW_SECTION_MIXING