Named text sections for caml_system__code_begin/end symbols

[gretay-js]

Place the symbols caml_system__code_begin and caml_system__code_end into their own named text sections .text.caml.caml_system__code_begin and .text.caml.caml_system__code_end, respectively, when function sections are enabled. Other symbols that mark begin and end of code segments are already placed their own named text sections.

Note that this PR changes the section names for caml_hot__code_begin and caml_hot__code_begin symbols: they are prefixed with .text.caml instead of .text only. The new names are consistent with the section names for other code segment markers. Linker scripts that use caml_hot__* symbols will need to be adjusted, but it's probably only used by ocamlfdo.

With these changes, the symbols appear in the correct order in the executable:

caml_hot__code_begin
caml_hot__code_end
caml_system__code_begin
caml_call_gc
...

instead of

caml_system__code_begin
caml_hot__code_begin
caml_hot__code_end
caml_call_gc
...

when linked with default linkers on Linux systems.
It makes debuggers happy again, as originally intended here:

ocaml/runtime/amd64.S

Lines 308 to 310 in ca64723

	G(caml_system__code_begin):
	ret /* just one instruction, so that debuggers don't display
	caml_system__code_begin instead of caml_call_gc */

)

[let-def]

I believe the patch is correct. Though I am not familiar with other platforms (I was not even aware of \name syntax on ARM64).

[gretay-js]

Thank you @let-def for your review. Does this PR need a Changes entry?

[gasche]

(Note for @lpw25: we also expect a backport in 4.12.)

[lpw25]

Cherry-picked to 4.12 as 0046bcb