PR26378, sections initialised only by linker scripts are always read/write

PR26378, sections initialised only by linker scripts are always read/write

This changes the initialisation of output sections so that it is
possible to create read-only sections fed only from linker script
BYTE, SHORT, LONG or QUAD.  That currently isn't possible even for one
of the well-known ELF sections like .rodata, because once a section is
marked read/write that sticks.  On the other hand if we start
read-only, well-known ELF sections end up read/write as appropriate.
For example .tdata will still be SHF_ALLOC + SHF_WRITE + SHF_TLS.

	PR 26378
	* ldlang.c (map_input_to_output_sections): Start with a read-only
	section for data statements.
	* testsuite/ld-elf/size-2.d: Adjust to suit.