Evaluate output section end symbol assignments in partial link

[parth-07]

Until now, the linker was not evaluating output section end symbol assignments in partial links. This commit fixes this behavior.

Resolves #479

[quic-seaswara]

This is a good start but I am not sure about what is its effect on symbol values that are specified in the output section.

cat > 1.c <<  \!
int foo(float x) { return 0; }
!

cat > s.t << \!
SECTIONS {
  .foo : {
    *(.text.foo)
    u1 = 0x100;
  }
}
!

/usr/bin/clang -c 1.c -ffunction-sections -fno-asynchronous-unwind-tables
#eld
ld.eld -r 1.o -o r.o -T s.t
readelf -s r.o | grep u1
ld.eld r.o -o out.eld && readelf -s out.eld | grep u1
#bfd
ld.bfd -r 1.o -o r.o -T s.t
readelf -s r.o | grep u1
ld.bfd r.o -o out.bfd && readelf -s out.bfd | grep u1
#lld
ld.lld -r 1.o -o r.o -T s.t
readelf -s r.o | grep u1
ld.lld r.o -o out.lld && readelf -s out.lld | grep u1

Please document this behavior.