Unbreak fiber for macOS PPC

[barracuda156]

@olk Remember we were wondering why context was fixed but fiber failed? I preprocessed sources and found the cause. Of course this could not possibly work.

[barracuda156]

Here are error we get with sources in fiber:

 /opt/local/bin/as -v -I . -arch ppc -o bin.v2/libs/fiber/build/darwin-12.2.0/release/threading-multi/visibility-hidden/algo/work_stealing.o bin.v2/libs/fiber/build/darwin-12.2.0/release/threading-multi/visibility-hidden/algo/work_stealing.s
Apple Inc version cctools-localbuild, GNU assembler version 1.38
bin.v2/libs/fiber/build/darwin-12.2.0/release/threading-multi/visibility-hidden/algo/work_stealing.s:2357:Parameter syntax error (parameter 1)

This is preprocessed source:

LVL172:
	.loc 22 66 21
; 66 "./boost/fiber/detail/spinlock_ttas.hpp" 1
	or 27,27,27
; 0 "" 2
	.loc 22 66 63

Bingo.

[barracuda156]

@olk Could you review this please?

[olk]

@olk Could you review this please?

Unfortunately I don't own a Mac or PPC system.

Did you run the unit tests in the subdirectory?

[barracuda156]

@olk Not yet, but here it is merely a matter of wrong assembler syntax. On macOS registers must be prefixed with r (or f for float), so the existing code fails to build, and cannot build unless this is fixed.
(I fixed the same bug in Catch2, in Seqan3 and elsewhere. Pretty common.)

[barracuda156]

@olk Could we merge this? Without it fiber simply does not build at all.

[olk]

ty