Allow LOAD FRAGMENT (#1552)

This was implemented in #736 but removed after discussion in #869.

Fixes #1537

test/link/load-fragment/base/a.asm

@@ -0,0 +1,9 @@
+SECTION "Test", ROM0[0]
+AA: db 1
+LOAD FRAGMENT "RAM section", WRAM0[$c000]
+BB: db 2
+ENDL
+CC: db 3
+LOAD FRAGMENT "RAM section", WRAM0
+DD: db 4
+ENDL

test/link/load-fragment/base/ref.out.bin

@@ -0,0 +1 @@
+


test/link/load-fragment/base/ref.out.sym

@@ -0,0 +1,5 @@
+; File generated by rgblink
+00:0000 AA
+00:0002 CC
+00:c000 BB
+00:c001 DD