It's the first file it tries to build for gcc. Basically the relevant part of the log is:
g++ -c -g -O2 -DIN_GCC -DCROSS_DIRECTORY_STRUCTURE -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -DHAVE_CONFIG_H -DGENERATOR_FILE -I. -Ibuild -I../../../gcc-4.9.1/gcc -I../../../gcc-4.9.1/gcc/build -I../../../gcc-4.9.1/gcc/../include -I../../../gcc-4.9.1/gcc/../libcpp/include -I../../../gcc-4.9.1/gcc/../libdecnumber -I../../../gcc-4.9.1/gcc/../libdecnumber/dpd -I../libdecnumber -I../../../gcc-4.9.1/gcc/../libbacktrace \
-DBASEVER="\"4.8.2\"" -DDATESTAMP="\"\"" \
-DREVISION="\"\"" \
-DDEVPHASE="\"\"" -DPKGVERSION="\"(GCC) \"" \
-DBUGURL="\"<http://gcc.gnu.org/bugs.html>\"" -o build/version.o ../../../gcc-4.9.1/gcc/version.c
/home/march/wa/esp/xtensa-toolchain-build/build-lx106/root/xtensa-lx106-elf/bin/as: unrecognized option '--64'
Makefile:3845: recipe for target 'build/version.o' failed
make[1]: *** [build/version.o] Error 1
make[1]: *** Waiting for unfinished jobs....
3744 translated messages, 5728 fuzzy translations, 185 untranslated messages.
7227 translated messages, 1763 fuzzy translations, 657 untranslated messages.
/bin/sh ../../../gcc-4.9.1/gcc/../move-if-change tmp-optionlist optionlist
echo timestamp > s-options
make[1]: *** wait: No child processes. Stop.
Makefile:3886: recipe for target 'all-gcc' failed
make: *** [all-gcc] Error 2
So the compiled xtensa gcc assembler doesn't have the --32/--64 options, since it's build for the different architecture.
./as --version
GNU assembler (GNU Binutils) 2.24
...
This assembler was configured for a target of `xtensa-lx106-elf'.
Native gcc-as however does have these options and i can run /usr/bin/as --64 no problem. So I think the issue is somewhere in config scripts picking up this parameter from my local configuration. I'll have a look a bit more into it.