Add symbols for start and end of embedded linker

Add __dlwrap_linker and __dlwrap_linker_end symbols for use by the
embedded linker trampoline to determine which load segments are
part of the embedded linker and should not be exposed to the
executable.

Use flags to name the sections to avoid colliding with __dlwrap_linker
when no section name can be determined.

Bug: 190084016
Test: build with musl
Change-Id: I51ccbf9c15a65c6194fa263b7504a598dbf4ad18