lto, Darwin: Fix offload section names.

lto, Darwin: Fix offload section names.

Currently, these section names have wrong syntax for Mach-O.
Although they were added some time ago; recently added tests are
now emitting them leading to new fails on Darwin.

This adds a Mach-O variant for each.

gcc/ChangeLog:

	* lto-section-names.h (OFFLOAD_SECTION_NAME_PREFIX,
	OFFLOAD_VAR_TABLE_SECTION_NAME, OFFLOAD_FUNC_TABLE_SECTION_NAME,
	OFFLOAD_IND_FUNC_TABLE_SECTION_NAME): Provide Mach-O syntax
	versions when the object format is Mach-O.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>