[test] Add self-tests for flsl()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[libc] Add inline assembly implementation of flsl() using BSR instruction

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[test] Add tests for 64-bit division

On a 32-bit system, 64-bit division is implemented using the libgcc
functions provided in __udivmoddi4.c etc.  Calls to these functions
are generated automatically by gcc, with a calling convention that is
somewhat empirical in nature.  Add these self-tests primarily as a
check that we are using the correct calling convention.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Fix __libgcc attribute for recent gcc versions

We observed some time ago (in commit 4ce8d61 "Import various libgcc
functions from syslinux") that gcc seems to treat calls to the
implicit arithmetic functions (e.g. __udivdi3()) as being affected by
-mregparm but unaffected by -mrtd.

This seems to be no longer the case with current gcc versions, which
treat calls to these functions as being affected by both -mregparm and
-mrtd, as expected.

There is nothing obvious in the gcc changelogs to indicate precisely
when this happened.  From experimentation with available gcc versions,
the change occurred sometime between v4.6.3 and v4.7.2.  We assume
that only versions up to v4.6.x require the special treatment.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[test] Check for correct -mrtd assumption on libgcc arithmetic functions

As observed in commit 082cedb ("[build] Fix __libgcc attribute for
recent gcc versions"), recent versions of gcc have changed the
semantics of -mrtd as applied to the implicit arithmetic functions.

It is possible for tests to succeed even if our assumptions about
gcc's interpretation of -mrtd are incorrect.  In particular, if gcc
chooses to utilise a frame pointer in the calling function, then it
can tolerate a temporarily incorrect stack pointer (since the stack
pointer will shortly afterwards be restored from the frame pointer
anyway).

Add tests designed specifically to check that our implementations of
the implicit arithmetic functions manipulate the stack pointer as
expected by gcc.

The effect of these tests can be observed by temporarily reverting
commit 082cedb ("[build] Fix __libgcc attribute for recent gcc
versions"): without this fix in place, the tests will fail on gcc 4.7
and later.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[libc] Add isqrt() function to find integer square roots

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[libc] Add flsll()

Signed-off-by: Michael Brown <mcb30@ipxe.org>