Relative DT_RPATH/DT_RUNPATH vs. sysroot capable ld.

Relative DT_RPATH/DT_RUNPATH vs. sysroot capable ld.

This fixes two problems.  First, the --sysroot option wasn't available
with a ld configured without --with-sysroot, a historical accident.
This led to people configuring binutils with --with-sysroot=/ in order
to enable sysroot support, which exposes a case where ld wrongly
prepends the sysroot to a relative path.

	PR ld/18992
	* ldmain.c (main): Always enable --sysroot.
	* emultempl/elf32.em (gld${EMULATION_NAME}_after_open): Don't
	prepend sysroot to relative rpath/runpath.