[Bug 200335] libcompiler_rt depends on libm symbols

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=200335

            Bug ID: 200335
           Summary: libcompiler_rt depends on libm symbols
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

/usr/lib/libcompiler_rt.a (on stable/10 after 10.1) has the following undefined
symbols, which are provided by libm:

U fabs
U fabsf
U fabsl
U fmax
U fmaxf
U fmaxl
U logb
U logbf
U logbl
U scalbn
U scalbnf
U scalbnl

GCC's libgcc{.a,_s.so} intentionally avoids relying on external libm
functionality. Presumably we haven't encountered an issue with this before for
a few reasons:

- static libgcc.a / libcompiler_rt.a is probably used much less often
- the complex division routines that introduce these dependencies are probably
not used often
- code that would generate a call to the complex division routines likely links
with -lm (although the link order might be wrong?)

Here's a similar issue in Chromium:
https://code.google.com/p/nativeclient/issues/detail?id=4019

This will be a problem if we want to use compiler-rt instead of libgcc_s.so.

For reference, the other (non-math related) undef symbols are:

U _GLOBAL_OFFSET_TABLE_
U __stack_chk_fail
U __stack_chk_guard
U __stderrp
U abort
U fflush
U fprintf
U mprotect
U sysconf

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