Reset LIBPATH to fix the build of MIPS nacl_helper_bootstrap on Ubuntu Precise

Reset LIBPATH to fix the build of MIPS nacl_helper_bootstrap on Ubuntu Precise

We reset LIBPATH in the same way that LINKFLAGS is reset.

Without this, I get the following:

$ ./scons sel_ldr --mode=dbg-host platform=mips32 sel_ldr --verbose
...
cc -o scons-out/dbg-linux-mips32/obj/src/trusted/service_runtime/nacl_bootstrap_munge_phdr scons-out/dbg-linux-mips32/obj/src/trusted/service_runtime/linux/nacl_bootstrap_munge_phdr.o -Lscons-out/dbg-linux-mips32/lib -Ltoolchain/linux_mips-trusted/sysroot/usr/lib -lelf
/usr/bin/ld: fatal error: toolchain/linux_mips-trusted/sysroot/usr/lib/libgcc_s.so: unsupported ELF machine number 8
collect2: ld returned 1 exit status
scons: *** [scons-out/dbg-linux-mips32/obj/src/trusted/service_runtime/nacl_bootstrap_munge_phdr] Error 1

I don't know why resetting LIBPATH wasn't needed for ARM too.

Note that Scons doesn't handle incremental builds correctly for this
parameter: once nacl_bootstrap_munge_phdr is present, if the fix is
undone Scons doesn't try to rebuild the file.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=2275
TEST=see above

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=10995

[bradn, 2013-03-14 22:50:31]

lgtm