Darwin: Fix i686 bootstrap when the assembler supports GOTOFF in data.

Darwin: Fix i686 bootstrap when the assembler supports GOTOFF in data.

When we use an assembler that supports " .long XX@GOTOFF", the current
combination of configuration parameters and conditional compilation
(when building an i686-darwin compiler with mdynamic-no-pic) assume that
it's OK to put jump tables in the .const section.

However, when we encounter a weak function with a jump table, this
produces relocations that directly access the weak symbol section from
the .const section - which is deemed illegal by the linker (since that
would mean that the weak symbol could not be replaced).

Arguably, this is a limitation (maybe even a bug) in the linker - but
it seems that we'd have to change the ABI to fix it - since it would
require some annotation (maybe just using a special section for the
jump tables) to tell the linker that this specific circumstance is OK
because the direct access to the weak symbol can only occur from that
symbol itself.

The fix is to force jump tables into the text section for all X86 Darwin
versions (PIC code already had this change).

gcc/ChangeLog:

2020-03-22  Iain Sandoe  <iain@sandoe.co.uk>

	* config/i386/darwin.h (JUMP_TABLES_IN_TEXT_SECTION): Remove
	references to Darwin.
	* config/i386/i386.h (JUMP_TABLES_IN_TEXT_SECTION): Define this
	unconditionally and comment on why.