Change the linker's heuristic for computing the entry point for binaries so that...

Change the linker's heuristic for computing the entry point for binaries so that shared libraries default to an entry point of 0.

	* ldlang.c (lang_end): When computing the entry point, only
	try the start address of the entry section when creating an
	executable.
	* ld.texi (Entry point): Update description of heuristic used to
	choose the entry point.
	testsuite/ld-alpha/tlspic.rd: Update expected entry point address.
	testsuite/ld-arm/tls-gdesc-got.d: Likewise.
	testsuite/ld-i386/tlsnopic.rd: Likewise.
	testsuite/ld-ia64/tlspic.rd: Likewise.
	testsuite/ld-sparc/gotop32.rd: Likewise.
	testsuite/ld-sparc/gotop64.rd: Likewise.
	testsuite/ld-sparc/tlssunnopic32.rd: Likewise.
	testsuite/ld-sparc/tlssunnopic64.rd: Likewise.
	testsuite/ld-sparc/tlssunpic32.rd: Likewise.
	testsuite/ld-sparc/tlssunpic64.rd: Likewise.
	testsuite/ld-tic6x/shlib-1.rd: Likewise.
	testsuite/ld-tic6x/shlib-1b.rd: Likewise.
	testsuite/ld-tic6x/shlib-1r.rd: Likewise.
	testsuite/ld-tic6x/shlib-1rb.rd: Likewise.
	testsuite/ld-tic6x/shlib-noindex.rd: Likewise.
	testsuite/ld-x86-64/pr14207.d: Likewise.
	testsuite/ld-x86-64/tlsdesc.rd: Likewise.
	testsuite/ld-x86-64/tlspic.rd: Likewise.
	testsuite/ld-x86-64/tlspic2.rd: Likewise.