LD: Always make a SEGMENT_START expression section-relative

LD: Always make a SEGMENT_START expression section-relative

Fix an issue with the SEGMENT_START builtin function where its result is
absolute when taken from the default supplied, and section-relative when
taken from a `-T' command-line override.  This is against documentation,
inconsistent and unexpected, and with PIE executables gives an incorrect
result with the `__executable_start' symbol.

Make the result of SEGMENT_START always section-relative then.

	ld/
	* ldexp.c (fold_binary): Always make the result of SEGMENT_START
	section-relative.
	* testsuite/ld-scripts/segment-start.d: New test.
	* testsuite/ld-scripts/segment-start.ld: New test linker script.
	* testsuite/ld-scripts/segment-start.s: New test source.
	* testsuite/ld-scripts/script.exp: Run the new test.