x86/build: Rework binary conversion for boot/{cmdline,reloc}.c

x86/build: Rework binary conversion for boot/{cmdline,reloc}.c

There is no need to opencode .got.plt size check; it can be done with linker
asserts instead.  Extend the checking to all dynamic linkage sections, and
drop the $(OBJDUMP) pass.

Furthermore, instead of removing .got.plt specifically, take only .text when
converting to a flat binary.  This makes the process invariant of .text's
position relative to the start of the binary, which avoids needing to discard
all sections, and removes the need to work around sections that certain
linkers are unhappy discarding.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>