testsuite, Darwin: Add support for Mach-O function body scans.

testsuite, Darwin: Add support for Mach-O function body scans.

We need to process the source slightly differently from ELF, especially
in that we have __USER_LABEL_PREFIX__ and there are no function start
and end assembler directives.  This means we cannot delineate functions
when frame output is switched off.

TODO: consider adding -mtest-markers or something similar to inject
assembler comments that can be scanned for.

gcc/testsuite/ChangeLog:

	* lib/scanasm.exp: Initial handling for Mach-O function body scans.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>